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 updraft. (A draft is simply a vertical movement of air.) This powerful updraft is called a mesocyclone.

A mesocyclone 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 mesocyclones down with it to the ground. This downdraft 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 downdrafts) 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 industry.

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.

Industry experts predict that if this pace of growth continues, by 2050, one-third of the world’s electricity needs could be met by wind.