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 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
.
