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What Causes Wind to Blow?

What causes wind to blow? The quick answer is that wind blows because of differences in atmospheric pressure . When there’s a difference in pressure, air moves from areas of high pressure to areas of low pressure, creating what we feel as wind.

What Causes Wind?

A more detailed explanation involved the interplay between temperature and pressure of air. The latitude , seasons , geography, and whether it’s day or night affect the wind.

Role of Temperature and Pressure

The Sun is at the heart of why wind blows. Solar radiation heats the Earth, but not uniformly. Equatorial regions receive more direct sunlight than polar regions, warming them more. Warm air is less dense and rises, creating a low-pressure area. In contrast, cold air is denser and sinks, creating a high-pressure area. Air moves from high-pressure to low-pressure areas in an attempt to equalize these differences, creating wind.

Other Factors That Cause Wind

Other factors also influence wind patterns:

Seasons : The tilt of the Earth’s axis of rotation causes seasons, which lead to non-uniform heating of the northern versus the southern hemisphere.
Day/Night Cycle : Since the Sun causes the heating that drives wind, it makes sense that the day/night cycle affects the air too. But, the relationship is not always straightforward. Days are often warmer than nights, but a sunny day can lead to cloud formation that insulates the ground and makes nighttime warmer.
Coriolis Effect : Earth’s rotation deflects moving air. In the Northern Hemisphere, air is deflected to the right, while in the Southern Hemisphere, it’s deflected to the left.
Friction: Obstacles like buildings and trees slow down the wind. This is why wind speeds are often higher at greater altitudes where nothing blocks the air.
Topography: The shape of the land, such as vast plains or deep valleys, channels or blocks winds. Topography alters wind speed and direction.

Wind is the result of the constant attempt to balance out temperature and pressure differences, influenced by factors ranging from global circulation patterns to local geographical features.

Global Circulation

The Earth’s rotation and its tilted axis complicate the flow of air. This results in a pattern known as the three-cell circulation:

Hadley Cell: Warm air at the equator rises and moves towards the poles. Around 30° latitude, this air cools, descends, and returns to the equator. This cell is responsible for the trade winds.
Ferrel Cell: This cell exists between about 30° and 60° latitudes. Air flows towards the poles, rises, and then returns toward the equator.
Polar Cell: Cold, dense air at the poles descends and flows towards the equator. Around 60° latitude, this air rises.

The boundaries between these cells and the Earth’s rotation create the planet’s prevailing winds, such as the trade winds, westerlies, and polar easterlies.

Local Winds

The geography of an area affects heating and air flow:

Land and Sea Breezes: Land heats up and cools down faster than water. During the day, the land gets warmer than the sea, causing the air over the land to rise. Cooler air from the sea rushes in to replace it, creating a sea breeze. At night, the process reverses. Land cools faster, causing the air above it to be denser. This draws the warmer air from the sea towards the land, creating a land breeze.
Mountain and Valley Breezes: Mountains can also influence local wind patterns. During the day, mountain slopes heat rapidly, warming the air, causing it to rise and draw cooler air from the valley, creating a valley breeze. At night, the slopes cool quickly. The cooler air descends into the valley, forming a mountain breeze.

Wind With Names

Various regions around the world have specific recurring wind phenomena that are so characteristic of those areas that they have distinctive names. Here are some of the more famous ones:

Mistral: A cold, dry wind in southern France that blows from the north or northwest into the Gulf of Lion.
Sirocco (or Scirocco): A hot, dry wind from the Sahara Desert that blows northward into southern Europe, particularly Italy.
Santa Ana Winds: Hot, dry winds that blow from the desert areas inward to the coastal regions of Southern California. They are notorious for exacerbating wildfires.
Chinook (or Snow Eater): A warm, dry wind on the eastern side of the Rocky Mountains in the US. The Chinook can cause rapid temperature increases, often making snow disappear quickly.
Harmattan: A dry and dusty trade wind that blows south from the Sahara into the Gulf of Guinea region in West Africa, typically between December and February.
Bora: A cold, dry wind blowing from the northeast onto the Adriatic Sea’s western coast, particularly affecting Croatia.
Levante: A warm, moist easterly wind in the western Mediterranean area, especially the southern parts of Spain.
Pampero: A cold wind from the west or southwest that blows over the pampas in Argentina.
Monsoon: Though often associated with rain, a monsoon technically refers to a seasonal prevailing wind that lasts for several months. The Indian Monsoon is the most well-known, bringing heavy rain to the Indian subcontinent during certain times of the year.
Föhn (or Foehn): A warm wind coming down the leeward side of mountain ranges, particularly in the Alps.
Zonda: A dry wind in Argentina that blows on the eastern slope of the Andes. It is comparable to the Chinook.
Gregale: A northeast wind in the western Mediterranean area, especially affecting the Malta region.
Berg Wind: Hot, dry wind descending the Great Escarpment of southern Africa to the coast.

These winds often have a significant influence on local weather patterns and play a role in shaping the culture and lifestyles of the regions where they blow.

Getis, Arthur; Getis, Judith; Fellmann, Jerome D. (2000). Introduction to Geography (7th ed.). McGraw-Hill. ISBN 978-0-697-38506-2.
Hahn, Douglas G.; Manabe, Syukuro (1975). “The Role of Mountains in the South Asian Monsoon Circulation”. Journal of the Atmospheric Sciences . 32 (8): 1515–1541. doi: 10.1175/1520-0469(1975)032<1515:TROMIT>2.0.CO;2
Jordan, Michael (1993). Encyclopedia of Gods: Over 2, 500 Deities of the World . New York: Facts on File. ISBN 978-0-8160-2909-9.
Makarieva, Anastassia; V. G. Gorshkov, D. Sheil, A. D. Nobre, B.-L. Li (February 2013). “Where do winds come from? A new theory on how water vapor condensation influences atmospheric pressure and dynamics”. Atmospheric Chemistry and Physics . 13 (2): 1039–1056. doi: 10.5194/acp-13-1039-2013

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The Way the Wind Blows

The rotation of the Earth forces winds into clockwise and counterclockwise motions, depending on air pressure.

By C. Claiborne Ray

Q. Why do winds travel clockwise around high-pressure centers and counterclockwise around low-pressure centers? Is it different in the Southern Hemisphere?

A. Wind travels from areas of high pressure to areas of low pressure. Thus, the direction of the surface wind in a high-pressure weather system is basically outward , away from the system toward lower pressure. The wind’s direction in a low-pressure system is inward.

In the Northern Hemisphere, the turning of the earth on its axis results in deflection of the wind to the right. This is called the Coriolis effect , after Gustave-Gaspard Coriolis, the French scientist who first described it in 1835.

If the wind is moving inward, its deflection to the right results in a counterclockwise motion. If the wind’s motion is outward, the result is a clockwise spiral.

The effect is the opposite below the Equator, in the Southern Hemisphere. But meteorological authorities emphasize that this effect is only reliably seen in large-scale air masses . Small whirlwinds can spin in either direction, just as water spiraling down a drain can go in either direction, no matter which hemisphere the observer is in.

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Why Does Wind Blow?

Watch this video to learn about wind! Click here to download this video (1920x1080, 107 MB, video/mp4).

Wind is a part of weather we experience all the time, but why does it actually happen? The air will be still one day, and the next, powerful gusts of wind can knock down trees. What is going on here?

The main cause of wind is a little surprising. It’s actually temperature . More specifically, it’s differences in temperature between different areas.

How would temperature differences make the wind blow?

The gases that make up our atmosphere do interesting things as the temperatures change. When gases warm up, the atoms and molecules move faster, spread out, and rise. That’s why steam coming off a pot of boiling water always goes upward. When air is colder, the gases get slower and closer together. Colder air sinks.

The sun warms up the air, but it does so unevenly. Because the sun hits different parts of the Earth at different angles, and because Earth has oceans, mountains, and other features, some places are warmer than others. Because of this, we get pockets of warm air and cold air.

Different temperatures lead to different pressures

Since gases behave differently at different temperatures, that means you also get pockets with high pressure and pockets with low pressure . In areas of high pressure, the gases in the air are more crowded. In low pressure zones, the gases are a little more spread out.

You might think that the warm air would lead to a higher pressure area, but actually the opposite is true. Because warm air rises, it leaves behind an area of low pressure behind it .

Here comes the wind!

Now we’re getting to the part where wind happens. Gases move from high-pressure areas to low-pressure areas. And the bigger the difference between the pressures, the faster the air will move from the high to the low pressure. That rush of air is the wind we experience.

But why does the air move at all?

You might be wondering why the air would move from high pressure to low pressure in the first place. This is something that happens in nature all the time: things always try to even out. It’s called diffusion .

Even people do it! When people get onto a bus, do they all sit on the same side of the bus first? Do strangers sit next to each other when there are plenty of open seats? No way. People want to spread out as much as possible.

Next time you feel the wind blow, think about where it’s going, and what temperatures and pressures are causing it to do that.

Want to know more about wind? Find out how hurricanes and tornadoes form. And then check out the simulations where you can create a hurricane and control a tornado .

It’s a wonderful world — and universe — out there.

Come explore with us!

Science News Explores

Explainer: winds and where they come from.

Here’s an explanation for how and why the wind blows

Temperature and pressure are critical factors in the creation of a windy day.

behindlens/iStockphoto

Thermal wind balance

Thermal wind is the first of four main types of atmospheric flow. The most complex type of wind, it drives weather systems across the globe. It’s born from differences in the temperatures between the equator and the poles.

Picture a column of air from the ground to the top of the troposphere (TRO-puhs-sfeer) — that layer of atmosphere in which we live. As the sun beats down on it, this air heats up and expands. That makes the top of the column rise. This is common near the equator. If a column of air cools, such as at the poles, it contracts and shrinks. That same stack of air — still weighing the same amount — will now be shorter and denser.

This means that imaginary surfaces of constant density slope down toward the poles. That slope isn’t constant. These lines rise up and down like bumps and wrinkles in a blanket, depending on local conditions. But the general downward slope allows masses of air to slide toward the poles.

Thermal wind is what is created as those masses flow down this slope, carrying heat away from the equator. Meteorologists refer to this natural movement of solar energy out of the equator as “poleward heat transport.” Without it, most folks living outside the tropics would be buried beneath a sheet of ice. The equator would also be hot as a furnace.

As sun-warmed air rises near the equator and begins to move toward the poles, it also starts to drift eastward. This is due to Earth’s spin. It swirls the air from west to east around the planet.

That poleward-moving air also speeds up — dramatically. This is because Earth is an oblique (Oh-BLEEK) spheroid. If you took horizontal slices of the planet, those slices would be widest at the equator and narrowest at the poles. As Earth’s radius “shrinks” as one approaches the poles, the air has to speed up. This is because the air gets funneled into a smaller and smaller path. As it does so, its flow rate increases. (This process is due to what’s known as the conservation of angular momentum. ) In the Northern Hemisphere, this makes the air flow to the right with increasing speed. This swirling action is known as the Coriolis force.

Earth’s rotation and the change in the planet’s radius mean that moving air will always want to turn a bit to the right in the Northern Hemisphere (and the opposite direction in the Southern Hemisphere). This affects everything. A football tossed from one end of a stadium to another will naturally deflect 1.26 centimeters (a half inch) to the right! It’s also why winds in the upper atmosphere are relatively weak near the equator. Closer to the mid-latitudes, they howl. They’ve curved so much to the right that they often are speeding eastward at an impressive clip.

The jet stream

This is how the jet stream forms. This current of air snakes around the planet at speeds greater than 322 kilometers (200 miles) per hour. It’s found winding its way directly overhead of the strongest temperature contrasts at the surface.

This temperature gradient creates a steep density “hill” in the atmosphere where the air quickly sloshes down. The more rapidly it moves, the more the northern jet stream curves east. It’s just like riding a bicycle down a hill: The steeper the slope, the faster you go.

But as the air moves poleward, it never actually gets to the poles. Instead, it curves to the right rapidly because of Earth’s rotation and that Coriolis force . As a result, the jet stream meanders as it circles the Earth in each hemisphere. In the North, it moves air west to east in a circle around the mid-latitudes (and the opposite in the Southern Hemisphere), changing its path from season to season.

Poleward of the jet stream, the atmosphere is turbulent. Dozens of “eddies” of high and low pressure rotate around the globe, dragging wacky weather with them. On the equator side, the flow is described as “laminar.” That means it’s relaxed, and not chaotic.

Along this temperature boundary, a fierce atmospheric battleground develops. Colliding air masses of different temperatures spin up cyclones and other severe weather. Indeed, that’s why meteorologists refer to the jet stream’s position as a “storm track.”

The position of the jet stream influences the type of weather a region encounters. Consider the Northern Hemisphere, for instance. From December through February, the sun doesn’t reach the North Pole. This allows an extensive dome of super-cold air to bank up nearby. Atmospheric scientists refer to this flowing pool of cold air and low pressure as the polar vortex. It swells in size during winter. And when this flow of cold air surges southward, it pushes the jet stream into southern Canada and the northern United States. That can bring seemingly endless snowstorms to the upper Midwest and Northeast during the dead of winter.

Geostrophic winds

In summer, the poles warm. This weakens the temperature gradient between these zones and the equator. The jet stream responds by retreating some 1,600 kilometers (a thousand miles) northward. Now, the weather in the lower 48 U.S. states calms down. Sure, scattered thunderstorms erupt from time to time. But there are no huge storm systems spanning 1,600 kilometers or more to influence day-to-day events. Instead, the weather becomes geostrophic (GEE-oh-STRO-fik) — meaning relatively tranquil .

Ordinarily, air would flow from high pressure to low pressure. It would move across a pressure gradient. So the driving force would be known as the pressure gradient force. But the Coriolis force is still at play. So as parcels of air try to move down the gradient, they’re tugged to the right in the Northern Hemisphere (and the opposite direction in the southern one). These two forces cancel out. Like a perfectly-matched game of tug-of-war, the air isn’t yanked in either direction. It just meanders slowly around large pressure systems.

As a result, the air ends up circling around high- or low-pressure systems without moving toward or away from them. Closer to the surface, the flow is slightly ageostrophic (meaning the winds are no longer in complete balance) , due to the effects of friction with things at or near the surface.

Other large-scale wind-balancing effects

Sometimes, however, a low-pressure system spins so fast that a third force develops. It’s the same outward shove you feel on a merry-go-round or a vehicle rounding a corner. This is centrifugal force.

Rings of air in constant balance between these two forces spin around a storm’s center indefinitely. Their rather constant distance from the center is due to what’s known as cyclostrophic (Sy-klo-STROW-fik) balance . This represents a harmony — complementary actions — of the pressure-gradient and centrifugal forces.

On rare occasions, the Coriolis, centrifugal and pressure-gradient forces can all counteract one another. This perfect trifecta marks what scientists call gradient wind balance. It’s not worth a lot of fanfare. It does, however, dictate which way air parcels will move along the outer edges of a cyclone, any spinning column of air.

Clearly, there are a lot of moving parts that control the way the wind blows.

Local winds

The last category of winds are the ones you experience every day. And they’re different depending on where you are. Head down to the beach, for instance. On sunny days in the afternoon, air over land warms and rises. Cooler air sitting above the ocean rushes in to coastal regions, filling the void caused by the air rising over land.

This generates a line of puffy little cumulus (KEWM-u-lus) clouds that die out after the sun sets. Along peninsulas like Florida, colliding sea breezes can result in convergent winds. These colliding air masses force pockets of moist air high up into the atmosphere, forming thunderstorms. That’s why folks in the Southeast always carry umbrellas, even on sunny mornings. The “self-destruct” sunshine routinely generates scattered afternoon boomers.

The same process that sparks these storms reverses overnight. Since the ground cools faster than the water, the direction of the flow of air reverses. Instead of a sea breeze, a “land breeze” develops. Now, storms move out from the land, to the ocean. That’s the reason many people along the Gulf Coast can enjoy gorgeous offshore displays of evening lightning.

Wind also can vary locally along stationary fronts . These are the very sharp boundaries between regions of warm and cold air. Sometimes, stationary fronts can become hung up in valleys. When they do, the warm and cold air masses — winds — can slosh back and forth. Like water and oil in a bowl, they don’t mix. Instead, they just push each other back and forth like angry ocean waves. This can trigger dramatic temperature swings within short periods of time.

One particularly noteworthy example came from the Black Hills of South Dakota on January 22, 1943. A stationary front had established itself along the foothills in the western part of the state. According to the local National Weather Service office in Rapid City, the temperature skyrocketed from -20° Celsius (-4° Fahrenheit) at 7:32 a.m. to 7.2 °C (45 °F) just two minutes later. That afternoon, as the front retreated, over a span of just 27 minutes the temperature plummeted 32.2 degrees C (58 degrees F).

Similar wild swings in the mercury were noted across that region throughout the afternoon. Motorists reportedly had trouble driving because their windshields would fog over — or even crack — when crossing between warm and cold pockets. (Imagine trying to dress for the weather that day.)

Regardless of where you are or what season it is, the wind holds a lot of information. Its direction, temperature and speed all offer valuable clues about the state of the atmosphere. Next time you’re outside, take a second to pay attention to Mother Nature. There’s a lot she has to tell you if you note what’s blowin’ in the wind.

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ENCYCLOPEDIC ENTRY

Wind is the movement of air caused by the uneven heating of the Earth by the sun.

Earth Science, Meteorology, Engineering, Geography, Physical Geography

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Wind is the movement of air caused by the uneven heating of the Earth by the sun. It does not have much substance—you cannot see it or hold it—but you can feel its force. It can dry your clothes in summer and chill you to the bone in winter. It is strong enough to carry sailing ships across the ocean and rip huge trees from the ground. It is the great equalizer of the atmosphere , transporting heat, moisture, pollutants, and dust great distances around the globe. Landforms, processes, and impacts of wind are called Aeolian landforms , such as sand dunes and Loess deposits, which are deposits of silt. Differences in atmospheric pressure generate winds. At the Equator, the sun warms the water and land more than it does the rest of the globe. Warm equatorial air rises higher into the atmosphere and migrates toward the poles. This is a low-pressure system . At the same time, cooler, denser air moves over Earth’s surface toward the Equator to replace the heated air. This is a high-pressure system . Winds generally blow from high-pressure areas to low-pressure areas. The boundary between these two areas is called a front . The complex relationships between fronts cause different types of wind and weather patterns . Prevailing winds are winds that blow from a single direction over a specific area of the Earth. Areas where prevailing winds meet are called convergence zones . Generally, prevailing winds blow east-west rather than north-south. This happens because Earth’s rotation generates what is known as the Coriolis effect . The Coriolis effect makes wind systems twist counter-clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. The Coriolis effect causes some winds to travel along the edges of the high-pressure and low-pressure systems. These are called geostrophic winds . In 1857, Dutch meteorologist Christoph Buys Ballot formulated a law about geostrophic winds: When you stand with your back to the wind in the Northern Hemisphere, low pressure is always to your left. (In the Southern Hemisphere, low-pressure systems will be on your right.) Wind Zones The Earth contains five major wind zones: polar easterlies , westerlies , horse latitudes , trade winds , and the doldrums . Polar Easterlies Polar easterlies are dry, cold prevailing winds that blow from the east. They emanate from the polar highs , areas of high pressure around the North and South Poles. Polar easterlies flow to low-pressure areas in sub-polar regions. Westerlies Westerlies are prevailing winds that blow from the west at midlatitudes . They are fed by polar easterlies and winds from the high-pressure horse latitudes, which sandwich them on either side. Westerlies are strongest in the winter, when pressure over the pole is low, and weakest in summer, when the polar high creates stronger polar easterlies. The strongest westerlies blow through the “Roaring Forties,” a wind zone between 40 and 50 degrees latitude in the Southern Hemisphere. Throughout the Roaring Forties, there are few landmasses to slow winds. The tip of South America and Australia, as well as the islands of New Zealand, are the only large landmasses to penetrate the Roaring Forties. The westerlies of the Roaring Forties were very important to sailors during the Age of Exploration , when explorers and traders from Europe and western Asia used the strong winds to reach the spice markets of Southeast Asia and Australia. Westerlies have an enormous impact on ocean currents , especially in the Southern Hemisphere. Driven by westerlies, the powerful Antarctic Circumpolar Current (ACC) rushes around the continent (from west to east) at about 4 kilometers per hour (2.5 miles per hour). In fact, another name for the Antarctic Circumpolar Current is the West Wind Drift. The ACC is the largest ocean current in the world, and is responsible for transporting enormous volumes of cold, nutrient-rich water to the ocean, creating healthy marine ecosystems and food webs. Horse Latitudes The horse latitudes are a narrow zone of warm, dry climates between westerlies and the trade winds. Horse latitudes are about 30 and 35 degrees north and south. Many deserts, from the rainless Atacama of South America to the arid Kalahari of Africa, are part of the horse latitudes. The prevailing winds at the horse latitudes vary, but are usually light. Even strong winds are often short in duration. Trade Winds Trade winds are the powerful prevailing winds that blow from the east across the tropics . Trade winds are generally very predictable . They have been instrumental in the history of exploration, communication, and trade. Ships relied on trade winds to establish quick, reliable routes across the vast Atlantic and, later, Pacific Oceans. Even today, shipping depends on trade winds and the ocean currents they drive. In 1947, Norwegian explorer Thor Hyerdahl and a small crew used trade winds to travel from the coast of Peru to the coral reefs of French Polynesia, more than 6,920 kilometers (4,300 miles), in a sail-powered raft. The expedition, named after the raft ( Kon-Tiki ) aimed to prove that ancient mariners could have used predictable trade winds to explore wide stretches of the Pacific. Trade winds that form over land (called continental trade winds) are warmer and drier than those that form over the ocean (maritime trade winds). The relationship between continental and maritime trade winds can be violent. Most tropical storms , including hurricanes , cyclones , and typhoons , develop as trade winds. Differences in air pressure over the ocean cause these storms to develop. As the dense, moist winds of the storm encounter the drier winds of the coast, the storm can increase in intensity. Strong trade winds are associated with a lack of precipitation , while weak trade winds carry rainfall far inland. The most famous rain pattern in the world, the Southeast Asian monsoon , is a seasonal, moisture-laden trade wind. Besides ships and rainfall, trade winds can also carry particles of dust and sand for thousands of kilometers. Particles from Saharan sand and dust storms can blow across islands in the Caribbean Sea and the U.S. state of Florida, more than 8,047 kilometers (5,000 miles) away. Dust storms in the tropics can be devastating for the local community. Valuable topsoil is blown away and visibility can drop to almost zero. Across the ocean, dust makes the sky hazy. These dust storms are often associated with dry, low-pressure areas and a lack of tropical storms. Doldrums The place where trade winds of the two hemispheres meet is called the intertropical convergence zone (ITCZ) . The area around the ITCZ is called the doldrums. Prevailing winds in the doldrums are very weak, and the weather is unusually calm. The ITCZ straddles the Equator. In fact, the low-pressure doldrums are created as the sun heats the equatorial region and causes air masses to rise and travel north and south. (This warm, low-pressure equatorial wind descends again around the horse latitudes. Some equatorial air masses return to the doldrums as trade winds, while others circulate in the other direction as westerlies.) Although monsoons impact tropical as well as equatorial regions, the wind itself is created as the ITCZ moves slightly away from the Equator each season. This change in the doldrums disturbs the usual air pressure, creating the moisture-laden Southeast Asian monsoon. Results of Wind Wind traveling at different speeds, different altitudes, and over water or land can cause different types of patterns and storms. Jet Streams Jet streams are geostrophic winds that form near the boundaries of air masses with different temperatures and humidity . The rotation of the Earth and its uneven heating by the sun also contribute to the formation of high-altitude jet streams. These strong, fast winds in the upper atmosphere can blow 480 kph (298 mph). Jet streams blow through a layer of the atmosphere called the stratosphere , at altitudes of 8 to 14 kilometers (5 to 9 miles) above Earth’s surface. There is little turbulence in the stratosphere, which is why commercial airline pilots like to fly in this layer. Riding with jet streams saves time and fuel. Have you ever heard someone talk about a headwind or tailwind when they are talking about airplanes? These are jet streams. If they are behind the plane, pushing it forward, they are called tailwinds. They can help you get to your destination more quickly. If the winds are in front of the plane, pushing it back, they are called headwinds. Strong headwinds can cause flight delays. Hurricane A hurricane is a giant, spiraling tropical storm that can pack wind speeds of over 257 kph (160 mph) and unleash more than 9 trillion liters (2.4 trillion gallons) of rain. These same tropical storms are known as hurricanes in the Atlantic Ocean, cyclones in the northern Indian Ocean, and typhoons in the western Pacific Ocean. These tropical storms have a spiral shape. The spiral (swirling counter-clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere) develops as a high-pressure area twists around a low-pressure area. The Atlantic Ocean’s hurricane season peaks from mid-August to late October and averages five to six hurricanes per year. Wind conditions that can lead to hurricanes are called tropical disturbances. They begin in warm ocean waters when the surface temperatures are at least 26.6 degrees Celsius (80 degrees Fahrenheit). If the disturbance lasts for more than 24 hours and gets to speeds of 61 kph (38 mph), it becomes known as a tropical depression . When a tropical depression speeds up to 63-117 kph (39-73 mph), it is known as a tropical storm, and is given a name. Meteorologists name the storms in alphabetical order, and alternate with female and male names. When a storm reaches 119 kph (74 mph), it becomes a hurricane and is rated from 1 to 5 in severity on the Saffir Simpson scale . A Category 5 hurricane is the strongest storm possible on the Saffir-Simpson scale. Winds of a Category 5 blow at 252 kph (157 mph). Hurricanes spin around a low-pressure (warm) center known as the “eye.” Sinking air inside the eye makes it very calm. The eye is surrounded by a violent circular “ eye wall .” This is where the storm’s strongest winds and rain are. Hurricane Ethel, the strongest hurricane in recorded history, roared across the Gulf of Mexico in September 1960. Winds were sustained at 260 kph (160 mph). However, Hurricane Ethel quickly dissipated . Although its winds ultimately blew as far north as the U.S. states of Ohio and Kentucky, by the time it hit the coastline of the U.S. states of Louisiana and Mississippi, the storm surge was only about 1.5 meters (5 feet). Only one person died as a result of Hurricane Ethel, and damage to buildings and boats was limited to less than $2 million. Hurricanes bring destruction to coastal ecosystems and communities. When a hurricane reaches land, it often produces waves that can reach 6 meters (20 feet) high and be pushed by high winds 161 kilometers (100 miles) inland. These storm surges are extremely dangerous and cause 90 percent of all hurricane deaths. The deadliest hurricane on record is the Great Hurricane of 1780. Although sophisticated meteorological equipment was not available at that time, winds may have reached 320 kph (200 mph) as the hurricane hit Barbados and other islands in the Caribbean Sea. This may have been enough to strip the bark from trees. More than 20,000 people died as a result of the hurricane as it made its way across Barbados, St. Lucia, Martinique, Dominica, Guadeloupe, Dominican Republic, Bahamas, Turks and Caicos, and Bermuda. Although it decreased in intensity, the hurricane was tracked through the U.S. state of Florida before dissipating in the Canadian province of Newfoundland. Hurricanes can be destructive in other ways. High winds can create tornadoes . Heavy rains contribute to floods and landslides, which may occur many kilometers inland. Damage to homes, businesses, schools, hospitals, roads, and transportation systems can devastate communities and entire regions. Hurricane Katrina, which blew through the Gulf of Mexico and into the southern U.S. in 2005, is the most expensive hurricane in recorded history. Damage to buildings, vehicles, roads, and shipping facilities is estimated at about $133.8 billion (adjusted for inflation). New Orleans, Louisiana, was almost completely devastated by Hurricane Katrina. New Orleans, as well as Mobile, Alabama, and Gulfport, Mississippi, took years to recover from the damage done to their structures and infrastructure . The best defense against a hurricane is an accurate forecast that gives people time to get out of its way. The National Hurricane Center issues hurricane watches for storms that may endanger communities, and hurricane warnings for storms that will reach land within 24 hours. Cyclones Cyclones blow through the Indian Ocean in the same way hurricanes blow across the Atlantic. Cyclones blow in with air masses from the east, often the South China Sea, or the south. The most powerful and devastating cyclone in recorded history was the 1970 Bhola Cyclone. Like Hurricane Katrina, the Bhola Cyclone was a Category 3 storm. Its winds were about 185 kph (115 mph) as it made landfall along the coast of the Bay of Bengal, in what is today Bangladesh. More than 300,000 people died, and more than a million were made homeless. Cyclone winds devastated fishing villages, and storm surges drowned crops. Economic damage from the Bhola Cyclone was more than $479 million, adjusted for inflation. Typhoon Typhoons are tropical storms that develop over the northwest Pacific Ocean. Their formation is identical to hurricanes and cyclones. Typhoons form as equatorial winds and blow westward before turning north and merging with westerlies around the mid-latitudes.

Typhoons can impact a wide area of the eastern Pacific. The islands of the Philippines, China, Vietnam, and Japan are the most affected. However, typhoons have also been recorded as far as the U.S. states of Hawaii and even Alaska. Typhoons are often associated with extremely heavy rainfall. The wettest typhoon ever recorded was Typhoon Morakot in 2009. Morakot devastated the entire island of Taiwan, with winds of about 140 kph (85 mph). Storm surges and floods caused by those winds, however, caused the most damage. More than 277 centimeters (109 inches) of rain drenched Taiwan, leading to 461 deaths and $6.2 billion in damage. Nor’easters and Blizzards A nor'easter is a strong winter storm combining heavy snowfall, strong winds , and very cold temperatures. It blows from the northeast along the East Coast of the U.S. and Canada. A strong nor’easter is called a blizzard . The U.S. Weather Service calls a storm a blizzard when the storm has wind speeds of more than 56 kph (35 mph) and low visibility. (Visibility is the distance that a person can see— blizzards , like fog, make visibility difficult and a task like driving dangerous.) The storm must go on for a prolonged period of time to be classified as a blizzard , usually a few hours. Blizzards can isolate and paralyze areas for days, especially if the area rarely has snowfall and does not have the equipment to clear it from the streets. The Great Blizzard of 1888 was perhaps the worst in U.S. recorded history. Winds of up to 72 kph (45 mph) whipped the East Coast from Chesapeake Bay to as far north as Nova Scotia, Canada. More than 147 centimeters (58 inches) of snow fell across the region, causing freezing temperatures and massive flooding as the snow melted. The Great Blizzard resulted in 400 deaths and $1.2 billion in damage. Monsoon A monsoon is a seasonal change in the prevailing wind system of an area. They always blow from cold, high-pressure regions. Monsoons are part of a yearlong cycle of uneven heating and cooling of tropical and mid-latitude coastal regions. Monsoons are part of the climate of Australia, Southeast Asia, and in the southwestern region of North America. The air over land is heated and cooled more quickly than the air over the ocean. During summer, this means warm land-air rises, creating a space for the cool and moist air from the ocean. As the land heats the moist air, it rises, cools, condenses , and falls back to Earth as rain. During the winter, land cools more quickly than the ocean. The warm air over the ocean rises, allowing cool land-air to flow in. Most winter monsoons are cool and dry, while summer monsoons are warm and moist. Asia’s winter monsoons bring cool, dry air from the Himalaya mountains. The famous summer monsoon , on the other hand, develops over the Indian Ocean, absorbing tremendous amounts of moisture. Summer monsoons bring warmth and precipitation to India, Sri Lanka, Bangladesh, and Myanmar. The summer monsoon is essential for the health and economies of the Indian subcontinent. Aquifers are filled, allowing water for drinking, hygiene , industry , and irrigation . Tornado A tornado , also called a twister, is a violently rotating funnel of air. Tornadoes can occur individually or in multiples, as two spinning vortexes of air rotating around each other. Tornadoes can occur as waterspouts or landspouts, spinning from hundreds of meters in the air to connect the land or water with clouds above. Although destructive tornadoes can occur at any time of day, most of them occur between 4 and 9 p.m. local time. Tornadoes often occur during intense thunderstorms called supercells . A supercell is a thunderstorm with a powerful, rotating up draft . (A draft is simply a vertical movement of air.) This powerful up draft is called a mesocyclone . A meso cyclone contains rotating drafts of air 1 to 10 kilometers (1 to 6 miles) in the atmosphere . When rainfall increases in the supercell , rain can drag the meso cyclones down with it to the ground. This down draft is a tornado . Depending on the temperature and moisture of the air, a tornado can last a few minutes or over an hour. However, cool winds (called rear flank down drafts ) eventually wrap around the tornado and cut off the supply of warm air that feeds it. The tornado thins out into the “rope-like” stage and dissipates a few minutes later. Most tornadoes have wind speeds of less than 177 kph (110 mph), and are about 76 meters (250 feet) across. They can travel for several kilometers before dissipating. However, the most powerful tornadoes can have wind speeds of more than 482 kph (300 mph) and be more than 3 kilometers (2 miles) across. These tornadoes can travel across the ground for dozens of kilometers and through several states. These violent storms occur around the world, but the United States is a major hotspot with about a thousand tornadoes every year. " Tornado Alley," a region that includes eastern South Dakota, southern Minnesota, Nebraska, Kansas, Oklahoma, northern Texas, and eastern Colorado, is home to the most powerful and destructive of these storms. The most extreme tornado ever recorded occurred on March 18, 1925. This “Tri-State Tornado ” sped for 338 kilometers (219 miles) through Missouri, Illinois, and Indiana. The tornado destroyed local communications, making warnings for the next town nearly impossible. The Tri-State Tornado killed 695 people in 3.5 hours. The best protection against a tornado is early warning. In areas where tornadoes are common, many communities have tornado warning systems. In Minnesota, for example, tall towers throughout neighborhoods sound an alarm if a tornado is near. Measuring Winds Wind is often measured in terms of wind shear . Wind shear is a difference in wind speed and direction over a set distance in the atmosphere . Wind shear is measured both horizontally and vertically. Wind shear is measured in meters per second times kilometers of height. Under normal conditions, the winds move much faster higher in the atmosphere , creating high wind shear in high altitudes. Engineers must consider an area’s average wind shear when constructing buildings. Wind shear is higher near the coast, for example. Skyscrapers must account for this increased wind by having a stronger foundation or being engineered to safely “sway” with the wind . The amount of force that wind is generating is measured according to the Beaufort scale . The scale is named for Sir Francis Beaufort, who established a system for describing wind force in 1805 for the British Royal Navy. The Beaufort scale has 17 levels of wind force. “0” describes conditions that are so calm that smoke rises vertically. “12” describes a hurricane , and “13-17” are reserved only for tropical typhoons , the most powerful and potentially destructive wind systems. An anemometer is a device for measuring wind speed. Anemometers are used with tornado data collectors, which measure the velocity , precipitation , and pressure of tornadoes . Tornadoes ’ strength is measured according to the Fujita scale . The scale has six categories that designate increasing damage. After the tornado has passed, meteorologists and engineers determine the tornado ’s strength based on its wind speed, width, and damage to vegetation and human-built structures. In 2007, the Enhanced Fujita Scale was established in the U.S.; it provides more specific effects of the tornado to determine its destructive power. The Enhanced Fujita Scale has 28 categories, with the strongest cataloging damage to hardwood and softwood trees. Hurricanes are measured using the Saffir-Simpson scale. In addition to tropical depressions and tropical storms , there are five categories of hurricanes . The most powerful, Category 5, is measured by winds whipping at 252 kph (157 mph). Tropical cyclones and typhoons are often measured using other scales, such as Japan’s Tropical Cyclone Intensity Scale, which measures a typhoon as winds at 118 kph (73 mph). Impact on Climate Wind is a major factor in determining weather and climate . Wind carries heat, moisture, pollutants, and pollen to new areas. Many daily weather patterns depend on wind . A coastal region, for instance, undergoes changes in wind direction daily. The sun heats the land more quickly than the water. Warm air above the land rises, and cooler air above the water moves in over the land, creating an inland breeze. Coastal communities are usually much cooler than their inland neighbors. San Francisco is a coastal city in “sunny California,” and yet the author Mark Twain noticed that “the coldest winter I ever spent was a summer in San Francisco!” Wind affects the climate of a mountainous area differently. Rain shadows are created as wind interacts with a mountain range. As wind approaches a mountain, it brings moisture with it, which condenses as rain and other precipitation before coming over the crest of the mountain. On the other side of the mountain, dry “downslope winds ” can speed through mountain passes at nearly 160 kph (100 mph). One of the most familiar of these downslope winds is the Föhn. Föhn winds —nicknamed “snow-eaters”—develop as air descends over the Alps, creating a warmer climate in central Europe. Winds also help drive ocean surface currents around the world. The Antarctic Circumpolar Current transports cold, nutrient-rich water around Antarctica. The Gulf Stream brings warm water from the Gulf of Mexico up the East Coast of North America and across the Atlantic to Northern Europe. Due to the Gulf Stream , Northern Europe enjoys a much warmer, milder climate than other areas at similar latitudes, such as the U.S. state of Alaska. Impact on Ecology Wind has the power to move particles of earth—usually dust or sand—in great quantities, and over far distances. Dust from the Sahara crosses the Atlantic to create hazy sunsets in the Caribbean. Winds transport volcanic ash and debris for thousands of kilometers. Winds carried ash from the 2010 eruption of Eyjafjallajökull, a volcano in Iceland, as far west as Greenland and as far east as Great Britain. The massive 1883 eruption of Krakatoa, an island volcano in Indonesia, had even more dramatic atmospheric results. Winds carried volcanic ash and debris high in the atmosphere across the globe. Europe endured years of cold, damp summers and pink sunsets. Wind ’s ability to move earth can erode the landscape. In some cases, this takes places in the desert, as sand dunes migrate and change shape over time. The wind can also pick up massive amounts of sand and “sandblast” rock formations into stunning sculptures. The Altiplano region of South America has dramatically shaped ventifacts —rocks carved by the wind -driven sand and ice. The wind ’s power to erode the land can be detrimental to agriculture. Loess , a sediment that can develop into one of the richest soils for farming, is easily swept up by wind . Even when farmers take precautions to protect it, the wind can erode up to 2.5 kilograms of loess per square meter (1.6 pound per square foot) every year. The most famous example of this devastating windstorm is probably the Dust Bowl of 1930s North America. Dust Bowl storms could reduce visibility to a few feet, and earned names like "Black Blizzards ." Millions of farmers, especially those in the U.S. states of Oklahoma, Arkansas, and Texas, lost their land when they were unable to harvest any crops. However devastating to the economy, wind is an important way plants disperse seeds . This form of seed dispersal is called anemochory . Plants that rely on anemochory produce hundreds and even thousands of seeds . Seeds are carried by the wind to distant or nearby places, increasing the spread of the plant’s genetics. Some of the most familiar seeds dispersed by the wind are those of the fuzzy dandelion. Wind Energy Wind has been used as a source of energy for more than a thousand years—it has pushed ships around the globe and been captured in windmills to pump water; it has turned giant stones to grind grains, make paper, saw logs, and crush ore. Today, most wind energy is used to generate electricity for homes, businesses, hospitals, schools, and in dustry . Wind is a renewable resource that does not directly cause pollution . Wind energy is harnessed through powerful turbines . Wind turbines have a tall tubular tower with two or three propeller-like blades rotating at the top. When the wind turns the blades, the blades turn a generator and create electricity . Often, wind turbines are collected in windy areas in arrays known as wind farms . Many wind farms have been established on mountains, in valleys, and offshore, as the air from the ocean interacts with land-air. Some people think wind turbines are ugly and complain about the noise they make. The slowly rotating blades can also kill birds and bats—but not nearly as many as cars, power lines, and high-rise buildings. The economic drawback to wind farms , however, is the wind itself. If it's not blowing, there's no electricity generated. Still, use of wind energy has more than quadrupled between 2000 and 2006. Germany has the most installed wind energy capacity, followed by Spain, the United States, India, and Denmark. Development is also growing quickly in France and China. In dustry experts predict that if this pace of growth continues, by 2050, one-third of the world’s electricity needs could be met by wind .

Age of Sail The ability of ships to sail with powerful trade winds helped determine the political and engineering history of the Age of Exploration, sometimes nicknamed the Age of Sail. Spanish, Portuguese, and British ships were quick, relatively easy to maneuver, and their large, complex series of sails exploited trade winds and southern westerlies to travel across the ocean. Their empires in Africa, eastern Asia, and the "New World" of North and South America blossomed in the 16th-19th centuries.

Anemoi Deities representing the winds play an important role in mythologies around the world. In Europe, ancient Greek myths refer to the Anemoi , or wind gods, as Boreas (north wind), Eurus (east wind), Notus (south wind), and Zephyrus (west wind). In Aztec mythology, the four wind gods were Mictlanpachecatl (north wind), Tlalocayotl (east wind), Vitztlampaehecatl (south wind), and Cihuatecayotl (west wind). Other mythologies recognize one supreme god of the wind: Enlil of Sumeria, Amun in ancient Egypt, Fujin in Japan's Shinto culture, Fei Lan of ancient China, and Vayu, the Hindu god of wind.

Extraterrestrial Winds The same forces that cause winds on Earthuneven heating by the sun and the planets rotationcause other planets to develop strong winds. Jupiters famous Great Red Spot is actually a centuries-old hurricane-like storm, swirling at around 644 kph (400 mph). The strongest winds in the solar system, however, belong to its outermost planet, Neptune. Neptunian winds whip at speeds up to 2,100 kph (1,300 mph). Extrasolar planets (those outside our solar system) have even faster winds. The extrasolar planets of 51 Pegasi have winds that blow 14,400 kph (9,000 mph)!

Gone with the Loo There are dozens of names for winds that blow through specific regions. Some, like the noreasters that blow from the northeast down the East Coast, are not creatively named. Here are some others: barber : cold, moisture-laden wind that freezes on contact with hair and beards. brickfielder : hot, dry wind that carries enormous amounts of red dust from the deserts of southern Australia. Cape Doctor : cold, dry wind from the southeast that sanitizes the city of Cape Town, South Africa. chinook : warming wind rushing eastward down the Rocky Mountains of Canada and the U.S. Coromuel : strong, warm wind that blows from afternoon to early morning through La Paz, Baja California, Mexico. The wind was named after British sailor Samuel Cromwell, whose name the locals could not pronounce. Hawk : strong, cool breeze blowing westward through Chicago from Lake Michigan. levant : strong winds that blow from the Atlantic Ocean through the narrow Strait of Gibraltar in the western Mediterranean Sea. The Levant, the Mideast region in the eastern Mediterranean, does not experience the levant. Loo : strong, hot summer wind that blows across northern India from the arid deserts to the west, and is only stopped by the arrival of the monsoon. The Loo is such a powerful ecological and cultural force that ice creams and sherbets are consumed to combat Loo-induced fatigue. November witch : hurricane-force winds that develop as cold Arctic air masses meet warm air from the Gulf over the Great Lakes. Pembrokeshire Dangler : area where prevailing winds converge and cause a line of cold rain and snow to dangle north-south across the Irish Sea. Santa Anas : hot, dry winds that blow from the deserts and mountains of inland California to the coast. Santa Anas are often responsible for spreading Southern Californias destructive wildfires, earning them the nickname murder winds. sirocco : wind that reaches hurricane speeds as it crosses the Mediterranean Sea to southern Europe. Siroccos carry tons of dust and sand throughout northern Africa, and contribute to wet weather as they reach Europe. squamish : fast-moving, cold wind that rushes down the narrow fjords of British Columbia, Canada.

Kamikaze Like the Protestant Wind, kamikaze were specific historical winds. Kamikaze--translated as divine winds--were major typhoons that destroyed the invading Mongolian Navy off the coast of Japan in the late 1200s. In the 20th century, kamikaze became the informal name for suicide attacks during World War II. The official name for kamikaze strategy is tokktai .

Protestant Wind The Protestant Wind refers to the lucky weather encountered by the British Navy of the 16th-17th centuries. Britain had just become a Protestant nation. The first Protestant Wind was a storm that destroyed the (Catholic) Spanish Armada off the coast of England in 1588. The second was westward winds across the English Channel, allowing Protestant William of Orange to invade England and depose James II, the last Catholic monarch.

Subway Storm The Great Blizzard of 1888 shut down roads and rails along the East Coast of the United States. Many people were confined to their homes for a week. The devastation and inconvenience led urban leaders to invest in the creation of the first subway system in the U.S., which opened in Boston, Massachusetts, in 1897.

Windy City Chicago, Illinois, has been nicknamed the Windy City for more than a hundred years. Chicago is a lakeside city that experiences cool breezes coming off Lake Michigan. It is not, however, any windier than most other cities. The nickname most likely came from Chicago's relationship with Cincinnati, Ohio, in the 19th century. Their industrial economies, as well as their baseball teams, were fiercely competitive. Cincinnati leaders dismissed Chicago baseball players and businesses as being insubstantial and meaningless, windy and full of hot air.

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Global Wind Explained

The illustration below portrays the global wind belts, three in each hemisphere. Note that the U.S. lies primarily in the Westerly Wind Belt with prevailing winds from the west. Each of these wind belts represents a "cell" that circulates air through the atmosphere from the surface to high altitudes and back again. The cells on either side of the Equator are called Hadley cells and give rise to the Trade Winds at Earth's surface. How do we explain this pattern of global winds and how does it influence precipitation?

Global wind belts inculding hadley cells, mid-latitude cells, polar cells, polar easteries, polar front, westerlies, trade winds, and equatorial low winds

We'll start at Earth's equator, where solar radiation is the highest year around. Air near the equator is warmed and rises because it is less dense (mass/unit volume) than the air around it as shown in Figure 21 below.

Solar radiation is pushed toward the equator and it then rises.

The rising air creates a circulation cell, called a Hadley Cell, in which the air rises and cools at high altitudes moves outward (towards the poles) and, eventually, descends back to the surface. The continual heating and rise of air at the equator create low pressure there, which causes air to move (wind) towards the equator to take the place of the air that rises. On the other hand, sinking air creates high pressure at the surface where it descends. A gradient of pressure (high to low) is formed that causes air to flow away from the high and towards the low pressure at the surface.

Hadley Cells are formed as the air rises, Rising air leads to low pressure while sinking air leads to high pressure

The Earth would have two large Hadley cells if it did not rotate. But, because it does rotate, the rotation of the Earth leads to the Coriolis effect. You should view the short video on this so-called "effect" or "force." ( The Coriolis Effect ). Without going into detail as to why rotation creates this apparent force, the Coriolis effect causes winds (and all moving objects) to be deflected:

to the right in the Northern Hemisphere
to the left in the Southern Hemisphere

The Coriolis effect causes winds to deflect as they travel within circulation cells and results in the two large hypothetical Hadley cells breaking into six smaller cells, which looks something like the diagram below (and the first figure in this series).

Diagram showing how Hadley cells are broken up as the earth rotates.

Ok, so, we now have some idea about the origin of global wind systems that result from pressure gradients at Earth's surface. How does this produce precipitation, and where? Precipitation occurs where moisture-laden air rises, either by heating at the equator or by running up and over a more dense air mass. As the rising air cools its capacity to hold water decreases (relative humidity increases) and, at some point, saturation with respect to water vapor is reached. Then, condensation--clouds and rain!

As air cools, it sinks. As rising air is replaced, warm air then rises.

The diagrams above and below portray just the Hadley cell circulation, that is driven by heating in the equatorial region. On the surface, wind moves away from high pressure (High) and toward low pressure (Low). Convergence occurs near the equator (winds blow in towards one another) and Divergence occurs under the descending air that forms high-pressure belts. The final figure (Figure 26) shows all six cells diagrammatically, along with the pressure variations at the surface of the Earth and zones of typical wet and dry belts. Note particularly the dry belts near 30 degrees North and South.

Same diagram as above...except divergent wind and convergent wind are on the bottom.

The strong winds of a microburst blew these trees down. Notice how they are all blown in the same direction. Credit: NOAA

Wind is air moving from a place that has higher pressure to one that has lower pressure.

Let’s say that you blow up a balloon but don’t tie a knot at the open end. You just pinch it closed with your fingers. Then, you relax your fingers a little and let some air flow out of the balloon. Have you ever thought about why the air comes out of the balloon instead of going in?

Air comes out because the pressure inside the balloon is higher than the pressure outside the balloon. Air molecules will spread out, spacing themselves far apart if they have the room. Air pressure isn’t exactly the same everywhere at the Earth’s surface because the Sun heats some places more than others. In the places where air is warmed most, it rises. This leaves a little less air near the Earth’s surface than in surrounding areas that didn’t get as warm. There is still air everywhere, but the air molecules are more widely spaced in some areas and more tightly packed in others. The places with more air molecules have higher pressure. The areas with fewer air molecules have lower pressure. Air whooshes into the area with lower pressure. That’s wind. So when you let the air out of a balloon, you are making a little bit of wind.

Wind Strength

Sometimes wind is just a light breeze and other times it is strong enough to blow the roofs off buildings. To describe the strength of wind, weather reports often use words like Light Wind, Strong Wind, and Gale. The Beaufort Scale assigns a number between zero and twelve to describe the strength of wind. Zero of the Beaufort Scale is complete calm. Twelve describes the powerful winds found in hurricanes. Winds are also described with the direction they blow. Easterly winds blow from the east, while westerly winds blow from the west.

Global Wind Patterns

There are steady winds that always blow in the same direction because of the pattern of how air moves through the atmosphere over the entire planet. For centuries sailors have depended on these predictable winds known as the Trade Winds, Westerlies, and Polar Easterlies. These winds turn to the right in the Northern Hemisphere and to the left in the Southern Hemisphere because of Earth’s spin, a phenomenon known as the Coriolis Effect.

Specialized Wind Types

Sometimes wind is very strong but lasts only a short time. For example, thunderstorms can create high winds including microbursts and tornadoes. While the wind from a microburst flows down and away from a thunderstorm, wind from a tornado flows up and into a thunderstorm. Monitoring tools such as Doppler radar and the Low-Level Windshear Alert System are used to spot microbursts and tornadoes.

Microbursts form when air, cooled rapidly within a storm, zooms downward at high speeds because it is more dense than the surrounding air. When it gets to the ground it spreads across the Earth’s surface as straight-line winds moving at speeds over 100 miles per hour. They only last a few minutes but can be deadly.
Tornadoes form when there is a difference in air pressure between the center of the tornado and its outer edge. The center has very low pressure and the outer edge has very high pressure, creating winds that can blow at over 200 miles per hour. Exactly what causes tornadoes to form is a topic of ongoing research. NCAR’s VORTEX2 field project has been chasing twisters to figure out how they form.
Why the Wind Blows (animated video)
What Is Weather?
The Highs and Lows of Air Pressure
Clouds and How They Form
A Global Look at Moving Air: Atmospheric Circulation

Earth’s Atmosphere

Air movement, lesson objectives.

List the properties of the air currents within a convection cell.
Describe how high and low pressure cells create local winds and explain how several types of local winds form.
Discuss how global convection cells lead to the global wind belts.
Chinook winds (Foehn winds)
high pressure zone
katabatic winds
land breeze
low pressure zone
mountain breeze
polar front
rainshadow effect
Santa Ana winds
valley breeze

Introduction

A few basic principles go a long way toward explaining how and why air moves: Warm air rising creates a low pressure zone at the ground. Air from the surrounding area is sucked into the space left by the rising air. Air flows horizontally at top of the troposphere; horizontal flow is called advection . The air cools until it descends. Where it reaches the ground, it creates a high pressure zone . Air flowing from areas of high pressure to low pressure creates winds. Warm air can hold more moisture than cold air. Air moving at the bases of the three major convection cells in each hemisphere north and south of the equator creates the global wind belts.

Air Pressure and Winds

Within the troposphere are convection cells ( Figure below ).

Global Winds and Precipitation

Besides their effect on the global wind belts, the high and low pressure areas created by the six atmospheric circulation cells determine in a general way the amount of precipitation a region receives. In low pressure regions, where air is rising, rain is common. In high pressure areas, the sinking air causes evaporation and the region is usually dry. More specific climate effects will be described in the chapter about climate.

Polar Fronts and Jet Streams

The polar front is the junction between the Ferrell and Polar cells. At this low pressure zone, relatively warm, moist air of the Ferrell Cell runs into relatively cold, dry air of the Polar cell. The weather where these two meet is extremely variable, typical of much of North America and Europe.

The polar jet stream is found high up in the atmosphere where the two cells come together. A jet stream is a fast-flowing river of air at the boundary between the troposphere and the stratosphere. Jet streams form where there is a large temperature difference between two air masses. This explains why the polar jet stream is the world’s most powerful ( Figure below ).

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News & features, winter center, news / weather news, what are high pressure systems and how do they contribute to our weather.

By Kyle Elliott , AccuWeather meteorologist

When the weather is dry, tranquil and nice, you can typically thank high pressure systems for keeping stormy and rainy weather at bay.

A high pressure system is essentially a clockwise flow of dry, sinking air that typically builds into a region behind a departing storm system. High pressure systems can be linked to the jet stream by finding areas where the jet bulges northward.

The jet stream is essentially an atmospheric river of air located at the level where jets cruise. Winds in the jet stream often reach 250 mph.

On the eastern side of a high pressure, winds coming out of the north generally drag chillier air southward from northern latitudes.

Winds blow out of the south on the western side of a high pressure and bring warmer air northward from more tropical climates.

The calmest conditions and sunniest skies are found near the center of a high pressure system, where air sinks most efficiently and warms as it does so.

Most frequently, high pressures move around the globe in a west-to-east manner. However, these systems can sometimes reverse course or “buckle” and stop over a region for as long as a couple of weeks.

It is when these systems become semi-permanent over southeastern Canada and/or Greenland during the winter months that the jet stream is forced southward into the central and eastern United States, as well as parts of Europe. This type of weather pattern can breed dangerous blizzards and lead to bitter cold snaps in both Europe and the eastern U.S.

“High pressure systems can help strengthen nor'easters by funneling moisture from the Atlantic Ocean into these storms," AccuWeather Senior Meteorologist Alex Sosnowski said.

"At other times, moisture can develop on the southern side of high pressure areas," he added. "During the winter, this moisture can help produce bands of heavy snow downwind of the Great Lakes."

Another example of when high pressures can spoil the weather occurs when fog forms and becomes trapped in valley locations for days at a time.

"Light winds and cold, moist air trapped near the ground lead to this persistent fog, which is common in the broad, extensive valleys of the western United States in the winter," according to Sosnowski.

Sosnowski mentioned that fog created in the same manner may take much of the day to burn off in the deep valleys of the Appalachians during autumn, especially in the months of September and October.

In the summer months, high pressure systems become better established and semi-permanent over the central Atlantic and Pacific oceans.

The high over the Atlantic, deemed the “Bermuda High” by meteorologists, can lead to dangerous, long-duration heat waves in the Interstate 95 corridor, from Boston to Washington, D.C., when it becomes abnormally strong or shifts closer to the Eastern Seaboard.

Two heat waves, one in August of 2001 and the other in July of 2011, that shattered records in the eastern U.S. were a direct result of an abnormally strong Bermuda High. Newark, New Jersey, set an all-time record high temperature of 108 degrees F on July 22, 2011, at the peak of that heat wave.

"These enormous and nearly stationary areas of high pressure can keep calm winds, clear skies and oppressively hot air in place for well over a week at a time, so the relatively high concentrations of vehicle and industry emissions from the Midwest to the Northeast have nowhere to go," AccuWeather Meteorologist and Air Quality Blogger Faith Eherts said. "As a result, an increasingly poor air quality situation can develop."

The Bermuda High also plays a key role in the movement of tropical storms and hurricanes in the Atlantic Basin.

Easterly winds to the south of the Bermuda High force many tropical systems on a westward course from the eastern Atlantic to the Caribbean Sea and/or Gulf of Mexico. The Bermuda High can ultimately put dangerous tropical cyclones on a collision course with the mainland of the U.S.

When the Bermuda High shifts to the east of its typical position near Bermuda, the Islands of Bermuda may instead be most prone to a direct hit from a tropical system. Southerly winds to the west of the high force tropical systems to veer northward more quickly, toward Bermuda, than if the high were directly over the islands.

The summer monsoon season in the southwestern U.S. is often triggered and/or enhanced by a high pressure system over the Plains.

"Trapped moisture that gets recycled on a daily basis can lead to thunderstorms over the Rockies and Southwestern states," Sosnowski said.

So, despite the fact that high pressure systems traditionally yield pleasant, sunny weather, there are many cases in which they can enhance or create an undesirable to potentially life-threatening situation.

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Do Winds Always Blow From High Pressure to Low Pressure?

When the sun shines on the Earth, temperature changes lead to pressure changes.

How Does Wind Affect Weather?

Wind plays an important role in the Earth's weather. The official fastest wind speed of 253 miles per hour occurred in 1996 during Cyclone Olivia in Australia. The unofficial fastest wind, 318 miles per hour as calculated by Doppler radar, happened during a tornado near Oklahoma City in 1999. Understanding what causes wind, especially these destructive winds, starts with understanding how the Sun heats the Earth's surface.

TL;DR (Too Long; Didn't Read)

Wind is generated when air moves from a high pressure system to a low pressure system. The greater the pressure difference, the stronger the wind. Temperature differences cause these pressure differences.

Energy from the Sun

The Sun's energy heats the Earth's atmosphere unevenly. At the equator the heating is relatively consistent, while the Sun's energy spreads out over a larger and larger area as latitude increases. This difference in energy distribution creates global wind patterns.

As the atmosphere heats, the warmer air rises which creates areas of lower pressure. The colder, denser air forming adjacent high pressure systems moves to fill in the space left by the rising warmer air. The warm air cools when it nears the top of the troposphere and sinks back toward the Earth's surface, creating convection currents in the atmosphere.

High pressure weather systems typically result from colder air patterns while low pressure weather systems generally result from warmer air patterns.

Coriolis Effect and Wind Direction

If the Earth didn't spin, the convection currents in the atmosphere could develop winds that would blow from the poles all the way to the equator. The Earth's rotation around its axis, however, causes the Coriolis effect . The spinning Earth deflects the wind from a straight line into a curve. The stronger the wind, the greater the curve.

In the northern hemisphere the deflection curves to the right. In the southern hemisphere the deflection curves to the left. Another way to consider the direction of the Coriolis effect is from the perspective of an astronaut floating directly above the North pole. A helium balloon released north of the equator would travel in a counterclockwise direction.

If the astronaut were above the South pole instead and the balloon was released south of the equator, the balloon would appear to travel in a clockwise direction.

Trade Winds, Westerlies and Polar Easterlies

Meanwhile, returning to the equator, the cooling air at the top of the column of rising air is pushed aside and begins to fall back to the Earth's surface. The Coriolis effect twists the rising and falling air nearest the equator into the pattern of wind called the trade winds. In the northern hemisphere the trade winds flow from the northeast to the southwest while in the southern hemisphere the trade winds flow from the southeast to the northwest.

The wind pattern in the mid-latitudes flows in the opposite direction, generally west to east. Weather patterns in the U.S. move from the west coast toward the east coast. These winds are called the westerlies .

Above 60°N and below 60°S latitude the wind tries to blow toward the equator, but the Coriolis effect twists the wind in the pattern called the polar easterlies .

Early explorers learned about these general patterns and used them to explore the world. These wind patterns provided a steady source of propulsion for sailing ships traveling from Europe and Africa to the New World and back again.

Temperature, Air Pressure and Wind

The pressure differences that make wind happen are caused by differences in temperature. Local wind patterns may seem to violate the global wind patterns, until examined in greater detail.

Land and Sea Breezes

Land areas heat and cool faster than water. During the day, land heats up which heats the air above the land. The warm air rising above the land pulls cooler air in from the water. At night the reverse process occurs.

Water holds temperature longer than land so the warmer air rises, drawing cooler air from over the land. This coastal pattern occurs with locally gradual or slight pressure differentials. Stronger pressure systems negate the slight land-water difference that causes these breezes.

Mountain and Valley Winds

A similar local phenomenon occurs in mountainous areas. Sun heats the ground which heats up the adjacent air. The warmed air rises and colder air farther from the ground moves in, pushing the warmer air up the mountain. At night, ground cooling chills the air adjacent to the ground.

The colder, denser air flows down the mountain. This air flow may become the concentrated breeze in canyons referred to as cold air drainage.

Tornadoes and Hurricanes

The extreme winds of tornadoes and hurricanes also result from pressure differences. The extremely small distance between the high-pressure outer layer and the low-pressure core can generate wind speeds exceeding 200 mph. The Beaufort Wind Scale rates these winds based on observed phenomena. (See References for the Beaufort Wind Scale)

How does wind work, what causes the differences in pressure that result..., what are the doldrums, what weather occurs during a high pressure system, hadley cell effects, cold front effects on wind direction, list three factors that affect wind direction, does barometric pressure rise or fall when it rains, what major wind belts affect our climate the most, what are the types of prevailing winds, what are the winter monsoons, what is the difference between a nor'easter & a hurricane, the stages of mid-latitude cyclones, types of breezes, barometric pressure vs. wind speed of a hurricane, wind speed vs. air pressure, what causes the clouds of a hurricane to spiral, the differences between northern & southern hemisphere.

Weather Explained: Weather Fundamentals
University of Illinois: Coriolis Force
National Geographic: Wind
National Center for Atmospheric Research: What is Wind?
National Center for Atmospheric Research: Wind

About the Author

Karen earned her Bachelor of Science in geology. She worked as a geologist for ten years before returning to school to earn her multiple subject teaching credential. Karen taught middle school science for over two decades, earning her Master of Arts in Science Education (emphasis in 5-12 geosciences) along the way. Karen now designs and teaches science and STEAM classes.

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How do high and low weather systems work?

If hot air rises why does it form 'highs' while cold air forms 'lows'? How do high and low weather systems work?

High pressure systems are often associated with hot weather in summer, but in winter they can bring cooler weather. (Source: iStockphoto)

Highs and lows

A high pressure system occurs where the air mass above the Earth is denser than in surrounding areas, and therefore exerts a higher force or pressure.

"In a high pressure system you have dense air subsiding downwards and pushing down at the surface, Because the surface air is being pushed down from the air above it spreads outwards, which is why we generally have fair weather [in mid-latitude regions] when we have high pressure systems".

In summer, high pressure systems are associated with warm day time temperatures in mid-latitude regions. However, during winter high pressure systems over the centre of Australia bring "dry, cooler conditions with rain along the southern portion," says Siems.

Air in high pressure systems moves in an anticlockwise direction (in the southern hemisphere), while air in low pressure systems moves in a clockwise direction due to the rotation of the Earth.

At the surface of the Earth air flows from high pressure systems into low pressure systems.

Air moves upwards in low systems and downwards in high systems

"When you have a low pressure system you're dragging air inwards and its being pulled upwards", explains Siems. As the warm humid air spirals upwards, it cools and clouds form.

In winter, these may be thick enough to give rain or snow, and explain why people living in the southern states associate low pressure systems with cold and sometimes stormy weather.

In summer, lows travelling over warm water in the tropics can produce cyclones.

"You can get lots of convection or upward air movement going on in the tropics" explains Siems.

During January and February it is also common to see 'heat lows' — a hot dome-shaped pocket of low pressure air — form in Central Australia caused by intense rapid solar heating of the land surface.

"A heat low is when you have so much heating over a broad area. The air is hot and it expands and this heating is deep enough that the surface pressure does register a low," explains Siems.

While not responsible for the latest heatwave, heat lows combined with highs in the Great Australian Bight usually bring drier conditions to the southern part of Australia.

Anatomy of the January heatwave According to the Bureau of Meteorology January's heatwave started well before January. Several extreme heat conditions were recorded between September to December. At the same time central South Australia received its lowest July to December rainfall and below average rain fell across almost all south eastern Australia. Then, from the end of December, a series of slow-moving high pressure systems in the Great Australian Bight blocked the arrival of the monsoon in the north and cooler air from lows in the south. "Combined with the delayed monsoon, [the blocking high systems] allowed tremendous heat to build up over the continent," says Dr Karl Braganza from the Bureau of Meteorology. To see how the heatwaves progressed check out BOM's special report on the January heatwave.

Associate Professor Steven Siems from the Weather and Climate Program, School of Mathematical Sciences, Monash University spoke to Annie Hastwell

Tags: environment , climate-change , weather

Published 31 January 2013

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How the Atmosphere and Winds Affect Your Flight

We have no control over the weather, yet it plays such an important part in our everyday lives. This is even more so if you're a pilot. The weather and sky conditions are to a pilot what the ocean and waves are to a sailor. Understanding the weather plays a massive part in ensuring that an aircraft gets safely from A to B. Wind, temperature, air pressure and visibility are all important factors in not only how the aircraft flies but also how the punctuality and comfort of your flight will be affected.

The Atmosphere

The atmosphere can be described as a gaseous mass surrounding and moving along with the Earth with no defined upper limit. At this point, it's important to appreciate that the atmosphere moves along with the Earth. You'll understand why later. It is made up of roughly 78% nitrogen, 21% oxygen and a 1% mixture of other gases including carbon dioxide, hydrogen and argon. The air that these gases create behaves very much like a liquid.

The atmosphere is divided up into five main layers but commercial aircraft fly in the bottom two layers — the troposphere, which sits closest to the surface, and the stratosphere, which sits just above it.

As you climb up into the troposphere, the air pressure, temperature and air density all decrease. For every 1,000 feet you climb, the air temperature will decrease by 2°C. This continues up for around 36,000 feet, where you run into the tropopause, which is the boundary layer before reaching the stratosphere. Here, the temperature stops falling with increased height and becomes a constant value.

I say "around" 36,000 feet because the level of the Tropopause varies from day to day, season to season and most importantly, depending on its location above the Earth. The tropopause is affected by the temperature of the air below it. The warmer the air, the higher the tropopause. As a result, it tends to be lower over the poles (around 25,000 feet) and higher over the equator (around 55,000 feet).The tropopause is important because it effectively acts as a lid, keeping most of the wether below it.

As these conditions can very around the world, for standardization pilots use the International Standard Atmosphere (ISA). Under ISA conditions, at a sea-level airfield, the temperature is 15°C, it decreases by 1.98°C per 1,000 feet and the air pressure is 1013 hectoPascals (hPa). Pilots then refer to the conditions on a given day as ISA +/- so many degrees.

So, now we know the structure of the atmosphere, we can look at what effect is has on aircraft and how it can affect your flight.

We are all aware of the effect wind has on us, but what actually is it? Wind is defined as 'the sustained horizontal movement of air' and is caused by changes in air pressure. When you watch the weather forecast on TV, you'll see the wind measured in mph. However, in aviation, it's measured in knots (kt) -- with 1kt equaling 1.15mph.

You've more than likely seen a chart like the one below, showing areas of high and low pressure. Air will naturally flow from areas of high pressure towards areas of low pressure — much like water will flow down a slope and fill in a lower lying area. It's this flow of air that we experience as wind.

Charts like this show pilots areas of high and low pressure (Image courtesy web.gccaz.edu)

The rings on the chart are called isobars and indicate areas of equal pressure. The closer the isobars are together, the greater the pressure change for a given distance, and therefore the stronger the wind. We also know that, in the northern hemisphere, air moves anticlockwise around areas of low pressure and clockwise around areas of high pressure. The opposite is true in the southern hemisphere.

As a result, by looking at a chart like the one above, you can tell which direction the wind is coming from and how strong it will be. Knowing what the wind is doing is one of the most important factors that pilots need to know for their flight.

Why is that important for pilots? First up, let us dispel a common myth. Contrary to popular belief, it is not the engines that make aircraft fly, it is the wings. The engines merely provide the forward acceleration.

That's because a wing works by air flowing over its surface. When the airflow reaches a certain speed, the wing starts to provide lift. When the lift generated is greater than the weight, the aircraft climbs into the air. The engines provide the driving force to create that airflow over the wing. As a result, the strength and direction (velocity) of the wind is of great importance to aircraft at all stages of flight, particularly during takeoff and landing.

On takeoff, if the wing requires 100kt of airflow over it (airspeed) to create enough lift to fly, in zero wind conditions the aircraft will have to accelerate up to 100kt over the ground in order to lift off. However, if there is a 20kt wind blowing into the nose of the aircraft, a headwind, it only has to accelerate up to 80kt groundspeed before it can lift off. In effect, the aircraft is starting off with +20kt before it starts to move.

Conversely, if there is a 20kt wind blowing from behind the aircraft, a tailwind, it's starting off with a -20kt wind. As a result, it has to accelerate up to 120kt groundspeed before it can achieve enough lift to become airborne. This extra speed means that the aircraft will need more runway before it reaches its takeoff speed. Consequently, you'll understand why pilots prefer to takeoff into the wind.

This is the same for landing. With a 20kt headwind, the aircraft may have an airspeed of 100kt but only has a groundspeed of 80kt. This would require a much shorter runway than if the aircraft had a 20kt tailwind and a resultant groundspeed of 120kt. Next time you're on the takeoff run, have a look out of the window for the windsock. You'll notice, more often than not, that it's pointing down the runway.

When the winds are really strong, the groundspeed of landing aircraft can get quite slow. Whilst this isn't a problem for pilots and the aircraft, it does cause some headaches for Air Traffic Control. As the aircraft are moving over the ground more slowly, it means that fewer will land each hour. This isn't a problem at small airports, but at places like London Heathrow, the knock-on effects can be quite significant.

To minimize the delays from strong winds, ATC has moved to a system called TBS — time-based separation.

If I told you that turbulence is caused by fluctuations in lift, it will be no surprise to you that turbulence is closely linked with wind velocity. As the aircraft flies through the moving air mass above the Earth (remember from the Atmosphere section above?), it is subject to constantly changing wind velocity. These constant variations create changes in the lift the wing is providing at any given time. When the wind increases, the lift increases. When the wind decreases, the lift decreases.

Multiply these changes hundreds of times a second, and that's how you get turbulence. The greater the changes in the lift, the bumpier it feels. For more on turbulence and why you shouldn't be worried about it, check out my previous article, A Passenger's Guide to Turbulence .

Low-Level Winds

You may have noticed that on windy days, the flight seems fairly smooth until the latter stages of landing and then it starts to get bumpy. This isn't by luck — it's to do with something called the "boundary layer".

Think of a fast-moving river and how the water flows, remembering from earlier that the air behaves very much like a fluid. In the middle of the river, the water flows nice and smooth, since it is not hindered by anything. However, at the edges, the water starts to get turbulent as it rushes past rocks and sticks close to the bank. It's the same in the air.

A few thousand feet up, the air can flow freely, just like in the middle of the river. Yet, when the wind gets closer to the ground, it starts to hit buildings, trees, hills and other obstructions. It's this contact that causes the low-level turbulence you experience in the early and late stages of flight.

Jet Streams and Upper Level Winds

If you've been on a flight across the Atlantic, you've more than likely heard your pilots mention "jet streams". But what are they on about and why are they important for your flight?

The World Meteorology Organization defines a jet stream as, " A strong narrow current concentrated along a quasi-horizontal axis in the upper Troposphere or Stratosphere, characterised by strong vertical and lateral wind shears and featuring one or more velocity maxima. The wind speed must exceed 60kts".

What does that all mean? Basically, a jet stream is a concentrated area of really fast-moving wind. That's it. So what causes them to occur where they do?

You'll remember from earlier that the tropopause varies in height above the surface depending where on the Earth you are. Over the poles it's lower, and over the equator it's higher. However, in both hemispheres at approximately 30° and 60°, there are steps in the tropopause where large temperature differences occur.

In these locations, due to the rapid change of temperature in a localized area, the wind also changes rapidly. It's these changes that create the jet streams. The greater the temperature difference, the stronger the wind. This is why jet streams tend to be faster in the winter when there is a greater range of temperatures between the colder polar air and the warmer air next to it.

Jet streams flow from west to east, which is why flights from Europe to North America take longer than the other way round. In the core of jet streams, winds regularly reach 100kts-150kts. Occasionally they can reach 150kts-200kts. As a result, aircraft routes across the Atlantic vary according to the wind. When going westbound, the routes keep aircraft out of the strongest headwinds. When heading east to Europe, they try to make the most of these jet streams by flying as close to the core as possible.

These strong winds also explain why it's often more bumpy on flights heading eastbound than heading west. As pilots, we try to fly in the very core of the jets. Here, like in the river example above, the air is much smoother. You can be rocketing along with a 150kt tailwind and be in completely smooth air. Descend just a couple of thousand feet and you can have half the tail wind and be bumped around all night.

For more on the difference between ground speed and air speed, check out this story .

Bottom Line

The weather plays a massive part in how your flight pans out -- whether it takes off on time, how bumpy it is and how long it will take. The properties of the atmosphere affect all the weather that is created within it. The wind, and how it behaves, has the biggest impact on your flight. From how bumpy it is to how likely it is to be delayed. Next week, I'll be explaining the other main parts of the weather that can affect your flight — thunderstorms and fog.

← Wind and Balanced Forces | Winds Around a High-Pressure Center →

Winds Around a Low-Pressure Center

There are two significant differences between the examples on Winds and Balanced Forces and the ones on this page:

1) There was only one wind used with the straight isobars (lines of constant air pressure), representing a single latitude. With low-pressure centers, there is a physical size to them, so latitude changes with location.

In the examples below, the outer winds are 500 km from the center. One degree latitude equals 111 km, so the distance between the northernmost wind and the southernmost is 9º latitude.

The magnitude and direction of the Coriolis force depend on the latitude, so Coriolis force is not uniform across the low-pressure system.

2) Winds tend to spiral into the center of the low-pressure system, so they curve and experience centrifugal force. Now we are dealing with four forces acting on the moving air: pressure gradient force, friction, Coriolis force, and centrifugal force.

In each of the animations that follow, data for the location at the bottom position is output. The site has a yellow circle around it. This point isn’t always the southernmost point. When the center latitude is 90ºS, its latitude is 85.5ºS, which is to the north.

Effect of Coriolis Force (Latitude)

The pressure center’s latitude starts at 90ºN and changes in 10º increments southward. The pressure gradient is 7 mb / 100 km toward the center, and the drag coefficient represents a forest creating friction with the moving air.

What patterns do you observe as latitude changes?

The angle the winds cross the isobars is called the cross-isobar angle .

1) The cross-isobar angle increases as the latitudes approach the Equator (0º), so the winds flow more directly toward the center.

2) Winds spiral counterclockwise toward the low-pressure center in the Northern Hemisphere and clockwise in the Southern Hemisphere.

3) Winds near the center are slower than the winds further from the center.

4) The cross-isobar angles are not constant across the low-pressure system. The winds closest to the center are deflected further from the pressure gradient (remember, always perpendicular to the isobars) than the winds further from the center.

5) Something odd is happening when the latitude of the pressure center is at the Equator (0º). Winds at 0º could not be calculated mathematically. Notice that the winds in the Northern Hemisphere (the upper half of the image) are turning counterclockwise, and the winds in the Southern Hemisphere (bottom half) are turning clockwise.

What cause these patterns?

1) The cross-isobar angle increases as the latitudes approach the Equator (0º) since lower latitudes have smaller values of Coriolis force for given wind speed. There is less turning away from the pressure gradient at lower latitudes.

2) Winds spiral in opposite directions north and south of the Equator because the Coriolis force is perpendicular to the right of the wind in the Northern Hemisphere and to the left in the Southern Hemisphere.

3) Winds near the center are slower than the winds further from the center because the winds turn more toward the center of the low. The tighter the turn, the higher the outward acting centrifugal force, which offsets a portion of the inward-directed pressure gradient.

4) Friction and Coriolis force are both proportional to wind speed, but friction decreases more as wind speed decreases. Since centrifugal force increases toward the center of the low-pressure center (see #3), the wind speed decreases, and friction decreases more than the Coriolis force. The cross isobar angle decreases toward the low-pressure center since there is proportionately more force turning the wind away from the pressure gradient.

5) When a low-pressure center is at or near the Equator, winds in the Northern Hemisphere are deflected to the right of the pressure gradient and those in the Southern Hemisphere to the left. Winds are no longer spiraling around the low-pressure center.

Effect of Pressure Gradient

The pressure gradient begins at 7 mb / 100 km toward the center and decreases in 0.5 mb / 100 km increments. The pressure center’s latitude is 45ºN, and the drag coefficient is representative of a forest creating friction with the moving air.

What patterns do you observe as the pressure gradient decreases??

1) The cross-isobar angle decreases as the pressure gradient weakens.

2) Wind speed decreases at the pressure gradient decreases.

4) The cross-isobar angle for the winds closest to the center are less than for the winds further from the center.

1) The cross-isobar angle decreases as the pressure gradient force weakens since friction decreases more than the Coriolis Force, so there is more turning away from PGF.

2) Wind speed decreases at the pressure gradient decreases since there is less push to get the air to move.

3) Winds near the center are slower than the winds further from the center since the radius of curvature is higher, which increases the centrifugal force that offsets a portion of PGF. Remember, the centrifugal force and PGF are perpendicular to the isobars, but they are in the opposite direction from each other.

4) The cross-isobar angle for the winds closest to the center are less than for the winds further from the center, which is also a result of the centrifugal force offsetting the PGF, and friction decreases more than the Coriolis force.

Effect of Friction (Drag Coefficient)

In these animations, the drag coefficient, which characterizes the roughness of the surface interacting with the wind) decreases from 0.03 (a heavily forested mountain) to 0 (no friction). The pressure gradient across the area is 7 mb / 100 km, and the low-pressure center’s latitude is 45ºN.

What patterns do you observe as friction decreases?

1) The cross-isobar angle decreases as the drag coefficient decreases (friction decreases). When frictionless, the winds are parallel to the isobars, so they are not converging toward the low-pressure center.

2) Wind speed increases as friction decreases.

4) Except for the frictionless case, the cross-isobar angles for the winds closest to the center are less than for the winds further away.

1) When friction is zero, the Coriolis and centrifugal forces must balance the pressure gradient force. Since the centrifugal force opposes the pressure gradient force, Coriolis bends the wind, so the sum of the Coriolis and centrifugal forces are equal and opposite the pressure gradient force. Since Coriolis is always perpendicular to the wind, the wind is parallel to the isobars.

2) Since friction opposes the motion of the wind, when there is less friction, there is more acceleration by the pressure gradient.

3) Winds near the center are slower than the winds further from the center since the tighter turning of the wind creates more centrifugal force, which opposes the pressure gradient. There is less net force to accelerate the wind closer to the center of the low.

4) Friction decreases more than Coriolis force as wind speed decreases (see #3). There is a higher amount of force to turn the wind away from the pressure gradient as the wind approaches the center of the low.

Role of Centrifugal Force on Low-Pressure Systems

Low-pressure centers create storms (see the Clouds section), and we tend to think of storms with strong winds. But centrifugal force actually slows the winds toward the center of the storm!

The centrifugal force also affects wind direction as air flows toward the center of a low (see the examples of the low-pressure systems centered at 10, 20, and 30ºN below). This creates a broader region of converging surface wind as storms form further from the Equator. This will be important to think about when exploring the Clouds and Precipitation sections.

Hurricanes and Latitude

Hurricanes (called typhoons in the Eastern Hemisphere) basically have circular isobars surrounding the low-pressure center. Interestingly, they don’t form within 3º of the Equator. Why?

A hurricane takes up space, so latitude changes from the northern tip to the southern. When close to the Equator, part of the low-pressure region will be in the Southern Hemisphere, and part in the Norther. The changing direction of the Coriolis force acting on the wind doesn’t allow the winds to spiral into the center of the low, which is an essential ingredient for a hurricane to form (see the example of a low-pressure center at 1ºN). There is enough room for this spiral flow to form by roughly 3º N or S.

As hurricanes move away from the Equator, Coriolis force increases proportionately for a given wind speed. As winds flow toward the center of the low, they are deflected away from the pressure gradient more and more (see the examples of lows at 10, 20 and 30ºN). Hurricanes tend to grow in size as they move to higher latitudes.

You can find the latest forecast on our website , by following us on Twitter and Facebook , as well as on our mobile app which is available for iPhone from the App store and for Android from the Google Play store . Keep track of current weather warnings on the weather warning page .

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Windswept commuters brave the wind and rain on Waterloo Bridge London

Strong winds and lightning strikes batter much of UK

Met Office says it could not verify if strong gusts in Newcastle-under-Lyme in Staffordshire were a tornado

Strong winds and hundreds of lightning strikes have battered much of England, Wales and Northern Ireland with a possible tornado reported in Staffordshire.

The Met Office said it could not verify if the strong gusts in Newcastle-under-Lyme were a tornado but added that the winds forecast on Monday morning “had some potential” to create one.

Pictures shared online appeared to show damaged fences and debris scattered across the street, while police said they attended a scene at St Giles Road in the town just before 7.45am after strong gusts damaged roofs.

The Met Office spokesperson Stephen Dixon said: “Tornadoes do happen in the UK and they’re generally short-lived in nature, but around 30 a year are reported on average. Today there have been some reports of some particularly impactful winds. The fronts that were moving southwards this morning had some potential for some short-lived tornadoes within them, but we would need to assess the impact.”

A large tree lies on its side next to a road

It was reported that forecasters had recorded at least 450 lightning strikes in England .

Bridges, parks and National Trust sites across the UK were forced to close due to the windy conditions. The M48 Severn Bridge was closed in the early hours of Monday morning in both directions, with road users advised to use the M4 Prince of Wales Bridge as an alternative.

In Yorkshire, the Humber Bridge was forced to close to wind-vulnerable vehicles including motorhomes, cars with roof boxes, motorcycles and high-sided heavy goods vehicles.

Parks were shut across London including sections of Hampstead Heath and West Ham Park and playgrounds in Regent’s Park, Greenwich Park and Hyde Park. Kew Gardens, also in London, announced on social media that it would not be opening on Monday due to “adverse weather conditions”.

Popular tourist sites including parts of Dover Castle and Ainsdale beach in Southport were also forced to close.

The Met Office issued a yellow warning for wind at 6.25am on Monday that covers the whole of Northern Ireland, Wales and England as far north as Cumbria until 10pm. Gusts of up to 45mph were expected inland and in exposed coastal areas there could be winds of up to 55mph with heavy rain showers.

On Monday temperatures were expected to be below average for this time of year, peaking at between 11C (51.8F) and 12C in southern England and between 8C and 11C in the north, with rain showers expected up until Friday, the Met Office said.

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It has forecast some high pressure over the weekend that could lead to drier weather.

Dixon said: “By the time we get to the weekend, there’s a signal for higher pressure to build in from the south-west which would settle things down in terms of rainfall totals.

“We’ve obviously seen a very wet few weeks and months indeed for the UK, but by the time we get to Saturday and into Sunday we’re looking at high pressure building in from the west, which would see a reduction in those rainfall totals and some much needed dry weather for some areas.”

The forecaster has said windy conditions are likely to cause delays to road, rail, air and ferry transport as well as a possible short-term power cuts.

Northern Ireland

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10.11: See how the gradient wind has a role in weather.

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\[V_{g}=-\frac{1}{f} \frac{\partial \Phi}{\partial n}\]

Replacing the pressure gradient force $\left(\frac{\partial \Phi}{\partial n}\right)$ with $-f V_{g}$ in the gradient balance equation results in an equation that relates these gradient velocities to the geostrophic velocity:

\[\frac{V^{2}}{R}+f V-f V_{g}=0 \quad\) or \(\quad \frac{V_{g}}{V}=1+\frac{V}{f R}\]

In a regular low (middle, figure below), R > 0 so that V g > V . The velocity in a curve around a low-pressure area is subgeostrophic .

In a regular high (right, figure below), R < 0 so that V g < V . The velocity in a curve around a high-pressure area is supergeostrophic .

Gradient balance in Northern Hemisphere. left: Geostrophic balance; center: regular low balance; right: regular high balance. Note that the PGF is independent of velocity but both the Coriolis force and the centrifugal acceleration are dependent on velocity. In the figure the geostrophic velocity is represented by v g and the gradient wind velocity is represented by v gr .

Credit: H.N. Shirer

Think of it this way. The pressure gradient force is independent of velocity and so is always there for a given geopotential gradient. In a regular low, the centrifugal and Coriolis forces, both dependent on velocity, sum together to equal the pressure gradient force, whereas for geostrophic flow, only the Coriolis force does. Thus, the velocity in the gradient balance case must be less than the geostrophic velocity for the same geopotential gradient.

So how do subgeostrophic and supergeostrophic flow affect weather?

Supergeostrophic flow around ridges and subgeostrophic flow around troughs helps to explain the convergence and divergence patterns aloft that are linked to vertical motions.

Look at the figure below, starting on the left. Going from geostrophic flow in the straight section to supergeostrophic flow at the ridge’s peak causes divergence aloft. This divergence causes an upward vertical velocity, which causes a low pressure area and convergence at the surface. As the air rounds the ridge’s peak, it slows down to become geostrophic, and then continues to slow down even more as the flow becomes subgeostrophic around the trough, thus causing convergence aloft. This convergence aloft causes a downward velocity, which causes high pressure and divergence at the surface.

So downwind of a trough is the favored location for divergence aloft, upward motion, and a surface low . Downwind of a ridge is the favored location for convergence aloft, downward motion, and a surface high. Since ridges form around high pressure aloft and troughs form around low pressure aloft, we see that the high aloft is offset relative to the surface low and the low aloft is offset relative to the surface high.

Thus subgeostrophic flow and supergeostrophic flow aloft are directly related to the formation of weather at the surface. Other factors like vorticity are also very important. The video below (1:09) describes how the gradient wind flow aloft can affect surface weather.

Trough Aloft Surface Low Video

Let's see how the gradient wind flow aloft can affect surface weather. Look at how the velocity changes as air flows around the ridge and then a trough aloft. Initially, the velocity is about geostrophic and straight-line flow. As it rounds the ridge, it speeds up. And then it slows down again to geostrophic in the straight section. As it goes through the trough, around the low pressure loft, it slows down to subgeostrophic and then speeds up to geostrophic in the next straight section. The speeding up causes divergence aloft. And the slowing down causes convergence aloft, just as you learned in lesson nine. You also saw how convergence aloft can lead to divergence at the surface. This contributes to a surface high. And how divergence aloft can lead to convergence at the surface, which contributes to a surface low. Thus, gradient flow contributes to surface weather. We often see a surface low forming on the downwind side of a trough.

Warming Winds

High pressure will keep fronts away from South Florida, but look for warming winds out of the Southeast to help temperatures gradually increase. By the weekend, it is going to feel more humid with highs climbing into the upper 80’s.

Last weekend highs were in the low 80’s, so temperatures will rise to above average values this upcoming weekend.

As far as rain is concerned, models keep our area very dry into next week. At best only a spotty shower possible with the best chance midweek.

Have a wonderful day South Florida and make it a safe one!

Vivian Gonzalez

Meteorologist, AMS Certified

WSVN Channel 7

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Why Does Wind Blow?
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The Highs and Lows of Air Pressure
Winds blow away from high pressure. Swirling in the opposite direction from a low pressure system, the winds of a high pressure system rotate clockwise north of the equator and counterclockwise south of the equator. This is called anticyclonic flow. Air from higher in the atmosphere sinks down to fill the space left as air is blown outward.
Explainer: Winds and where they come from
Contracting air creates zones of "low pressure.". They pull nearby air inward. That's why the wind blows: It moves from regions of high pressure to those where pressure is lower. The zone between the high- and low-pressure areas is known as a pressure gradient, or a zone over which the pressure varies from high to low.
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The Coriolis effect causes some winds to travel along the edges of the high-pressure and low-pressure systems. These are called geostrophic winds. In 1857, Dutch meteorologist Christoph Buys Ballot formulated a law about geostrophic winds: When you stand with your back to the wind in the Northern Hemisphere, low pressure is always to your left ...
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This process repeats and creates a pattern of high and low-pressure bands from the equator to the poles. We know that air flows from areas of high pressure to areas of low pressure. This creates air occurrence or winds. Now you might think that these winds would blow in straight lines from high to low-pressure areas.
Wind flow
The greater the difference between the pressures (the pressure gradient), the stronger the wind. In reality, wind does not flow directly from areas of high to low pressure as there is a separate force at work - the Coriolis effect. The rotation of the Earth causes wind to experience an apparent force known as the Coriolis force.
1.5: Lab 5
So, these high altitude winds typically run parallel to the lines of equal pressure, referred to as isobars. However, within about 30 meters of the surface, friction acts to slow the movement of the wind, countering the force of pressure gradient and the Coriolis effect, thus causing the wind to travel diagonally over the isobars, away from a ...
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4.3: Controls over wind direction and speed
The speed of the wind is controlled by the strength of the pressure gradient: the stronger the pressure gradient the higher the wind speed. The strength of the pressure gradient can be discerned from the spacing of isobars on a weather map. Figure 4.3.1 4.3. 1 shows two different pressure gradients. The distance between points A and B and C and ...
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A high pressure system is essentially a clockwise flow of dry, sinking air that typically builds into a region behind a departing storm system. High pressure systems can be linked to the jet ...
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Do Winds Always Blow From High Pressure to Low Pressure?
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How do high and low weather systems work? › Ask an Expert (ABC Science)
Highs and lows. A high pressure system occurs where the air mass above the Earth is denser than in surrounding areas, and therefore exerts a higher force or pressure. "In a high pressure system ...
How the Atmosphere and Winds Affect Your Flight
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Cyclone
cyclone, any large system of winds that circulates about a centre of low atmospheric pressure in a counterclockwise direction north of the Equator and in a clockwise direction to the south. Cyclonic winds move across nearly all regions of the Earth except the equatorial belt and are generally associated with rain or snow. Also occurring in much the same areas are anticyclones, wind systems ...
10.9: Global Atmospheric Circulations
The global wind belts are enormous and the winds are relatively steady. These winds are the result of air movement at the bottom of the major atmospheric circulation cells, where the air moves horizontally from high to low pressure. Technology today allows anyone to see global wind patterns in real-time, such as Earth Wind Map.
Winds Around a Low-Pressure Center
As winds flow toward the center of the low, they are deflected away from the pressure gradient more and more (see the examples of lows at 10, 20 and 30ºN). Hurricanes tend to grow in size as they move to higher latitudes. A low-pressure center located at 1ºN. A bit more than half of the system is above the Equator, and the rest is south.
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10.11: See how the gradient wind has a role in weather
The velocity in a curve around a high-pressure area is supergeostrophic. Gradient balance in Northern Hemisphere. left: Geostrophic balance; center: regular low balance; right: regular high balance. Note that the PGF is independent of velocity but both the Coriolis force and the centrifugal acceleration are dependent on velocity.
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