Local Winds

Mesoscale winds are winds that blow across areas of the surface ranging from a few miles to a hundred miles in width. Mesoscale winds are better known as local winds or regional winds. A local wind can persist anywhere from several minutes to several days. Local winds can be driven by temperature and pressure differences or by variations in topography, the shape and height of Earth's surface features. Regardless of their cause, local winds have certain general characteristics. In addition, local winds have many interesting and descriptive names.

Sea and land breezes

Sea breezes and land breezes are two familiar categories of local winds that are driven by differences in temperature and in air pressure, which is the pressure exerted by air on a given area. A sea breeze blows from the water to the shore. It comes as a welcome relief to coastal inhabitants on a hot day. A land breeze, in contrast, blows from the shore toward the water. Land breezes occur in the evening.

Sea breezes form on hot days because the land warms more rapidly than does the water. As a result, a low-pressure area develops over warm ground and a high-pressure area over the cooler water. A mild wind blows across the pressure gradient, from the high-pressure area (over the water) to the low-pressure area (over the land). At night, the process is reversed as the land loses heat more quickly than does the water. The resultant land breeze flows from the high-pressure area, over the shore, out to the low-pressure area, over the water.

Sea breezes and land breezes are strongest at the shoreline, where temperature and pressure differences are most pronounced. They lose intensity as they travel inland or out to sea. The time of day when temperature and pressure differences are greatest, and therefore the breezes are strongest, is midafternoon, when land reaches its maximum

WORDS TO KNOW

adiabatic process:

a process by which the temperature of a moving air parcel changes, even though no heat is exchanged between the air parcel and the surrounding air.

air pressure:

the pressure exerted by the weight of air over a given area of Earth's surface. Also called atmospheric pressure or barometric pressure.

anabatic wind:

winds caused by warm air close to Earth's surface. The air is less dense than the surrounding air and travels upward along a slope.

anemometer:

an instrument that measures wind speed.

barchan dune:

a sand dune that, when viewed from above, resembles a crescent moon, with the tips of the crescent pointing downwind. Also called barchane dune, barkhan dune, or crescentic dune.

chinook wall cloud:

a solid bank of wispy, white clouds which appears over the eastern edge of the Rocky Mountains in advance of a chinook wind.

cold front:

the leading edge of a moving mass of cold air.

compressional warming:

an adiabatic process by which an air parcel warms as it descends. The descending parcel is compressed by the increasing pressure of the surrounding air, which adds kinetic energy to the molecules. Also called compressional heating.

conduction:

the transfer of heat from a substance at a higher temperature to a substance at a lower temperature through the transfer of kinetic energy from a faster-moving molecule to a slower-moving molecule.

cumuliform:

a puffy, heaped-up cloud formation.

cyclone:

a weather system characterized by air that flows inward and circulates around a low-pressure area.

desert pavement:

hard, flat, dry ground and gravel that remain after all sand and dust has been eroded from a surface.

downburst:

an extremely strong, localized down-draft beneath a thunderstorm that spreads horizontally when it hits the ground, destroying objects in its path.

heat stroke:

a life-threatening condition that sets in when heat exhaustion is left untreated and the body has spent all its efforts to cool itself. Also called sunstroke.

hurricane-force wind:

sustained winds greater than 74 mph (119 kph).

katabatic wind:

a strong wind that travels down a mountain under the force of gravity, and is stronger than a valley breeze.

lake breeze:

a wind similar to a sea breeze that can be felt at the edge of a large lake.

leeward slope:

the slope of a mountain opposite to the direction of local or prevailing winds down which cold air descends, producing dry conditions.

lightning:

a short-lived, bright flash of light during a thunderstorm that is produced by a 100-million-volt electrical discharge in the atmosphere.

local winds:

winds that blow across surface areas ranging from a few miles to about 100 miles (about 160 kilometers) in width. Also known as mesoscale winds or regional winds.

mesoscale winds:

winds that blow across surface areas ranging from a few miles to about 100 miles (about 160 kilometers) in width. Also known as local winds or regional winds.

temperature. Sea breezes are generally stronger than land breezes, since temperature contrasts are greater during the day than at night.

While the strongest sea breezes blow in from over the ocean, sea breezes can also be felt on the edges of large lakes, such as the Great Lakes. They are called lake breezes. They form in the Northern Hemisphere, that is, north of the equator, between May and August on summer days when the land becomes warmer than the water. As a lake breeze or sea breeze travels over the land it acts like a cold front, or the leading edge of a moving cold air mass, thrusting the warm air above it. Vertical clouds, which are clouds developed upward to great heights, and rain showers are often produced over a stretch of land several miles (kilometers) inland.

Mountain and valley breezes

Mountain breezes and valley breezes arise from a mechanism similar to that of sea breezes and land breezes: different rates of surface heating and a resultant pressure differential. Valley breezes, also called anabatic (pronounced an-uh-BAT-ick; Greek for "climbing") winds are formed during the day. The sun warms air closest to the surface of a mountain most rapidly. Air that is farther from the surface, at the same altitude, warms more slowly. The warmer air hugging the surface, which is less dense than the surrounding air, travels upward along the slope. This type of wind is called a "valley breeze" because the air is flowing upward, out of the valley.

A valley breeze typically begins shortly after sunrise. It is strongest on clear, sunny days, and first develops on slopes that face east, toward the rising sun. Since east-facing slopes are also the first ones during the day to end up in the shade, the valley wind there generally stops by late afternoon.

South-facing slopes receive the greatest amount of sunlight throughout the day. Hence it is on these slopes that the strongest valley winds are found. If a valley wind contains sufficient moisture, it will produce cumuliform clouds, which are puffy, piled-up formations, and possibly showers over the mountains in the early afternoon.

At night, the temperature gradient between the surface air and the layer of air above it is reversed, forming a mountain breeze that blows down the slope. Once the sun goes down, the mountain surface begins losing heat to the atmosphere by radiational cooling, which is the movement of heat from the ground upward. The layer of air just above the surface loses heat to the surface in a transfer process called conduction and also cools rapidly. This cold, dense surface air travels down the mountain and sinks into the valley. A mountain breeze is also called gravity wind or drainage wind.

Mountain breezes are usually stronger than valley winds, since at night the temperature difference is greatest between the layers of air next to and those farther away from the mountain. This is especially true in the winter, when the ground cools very quickly at night. A mountain breeze is one type of katabatic wind (pronounced kat-uh-BAT-ick; Greek for "going down"), a wind that blows downhill.

Katabatic winds

A katabatic wind is any wind that travels down a mountain under the force of gravity. However, the term is usually reserved for downhill winds which are considerably stronger than mountain breezes.

As a katabatic wind descends the mountainside, it is warmed by compressional warming. Compressional warming occurs as an air parcel descends and is compressed by the increasing pressure of the surrounding air. That compression leads to an increase in the kinetic energy of molecules and a resulting increase in the temperature of the air. In cases where the air is very cold when it begins its descent, it may still be colder than the surrounding air when it reaches the base of the mountain. However, where the air was somewhat less cold to start with, the wind that flows into the lowland may actually be warmer than the air mass it is replacing.

Katabatic winds range in strength from gentle to hurricane-force. Their speed depends largely on the terrain over which they travel. For instance, the wind accelerates when it travels down long, steep slopes or is squeezed through narrow canyons and valleys.

Cold katabatic winds

Cold katabatic winds usually arise during winter or early spring on snow-capped mountains or high-elevation plateaus. The snow keeps the air above it exceedingly cold, forming a dome of high pressure just above the surface. The heavy, dense air descends along the mountainside and through the canyons. If a storm (a low-pressure system) moves into the area, the contrast in pressure between the cold surface air and the surrounding air increases, causing the wind to rush down the slopes even faster.

One of the most famous katabatic winds is the "mistral" of southern France. Its name comes from the Latin word magistral, which means "master wind." This cold, dry wind comes from the north or northwest in the winter. It originates in the snowy Alps and travels down to the Gulf of Lyons on the Mediterranean Sea.

As the mistral descends through the Rhône River Valley, it is squeezed through narrow passages and picks up speed. The gusts of a mistral can exceed 100 mph (160 kph), bringing a blast of frigid air to the otherwise warm French Riviera. The mistral sometimes lowers temperatures so much that frost forms and endangers vineyards.

A cold winter wind that occurs in eastern Europe is called the "bora." The bora blows from the Dinaric Alps down to the Adriatic Coast. This blustery wind travels from the north or northeast, like the mistral, and can also reach speeds greater than 100 mph (160 kph).

A "papagayo" (pronounced pa-puh-GUY-oh) is a strong, northeasterly wind that affects the Pacific coast of Central America, from Guatemala to the Gulf of Papagayo in Costa Rica. The papagayo winds are produced by a cold air mass that travels down through the Central American mountains. It brings weather that is cold and blustery, yet clear.

Experiment: How canyons affect wind speed

This experiment simulates how the wind accelerates as it squeezes through narrowing canyons or valleys. The only equipment needed is an electric fan, two pieces of cardboard or other material that can serve as walls, a pencil, and some string or yarn.

First, tape several 1-foot-long (30-centimeter-long) pieces of string or yarn to one end of the pencil. Then place the fan on a table and turn it on the lowest setting. Hold the pencil (by the end without the strings) a couple of feet (less than 1 meter) in front of the fan. Note how high the breeze blows the strings.

Now it's time to create your canyon walls. To do this, stand your pieces of cardboard on the table. Prop them up with bricks, heavy books, or other sturdy objects, if necessary. In order to "funnel" the wind, the distance between the walls must be greater at the end nearest the fan and smaller at the end farthest from the fan.

Turn the fan on the lowest setting again. Hold the pencil so that the end with the strings is just beyond the small opening between the "walls." The pencil should be about the same distance from the fan as it was the first time. Observe how high the strings are blown. Notice any difference in the intensity of the unobstructed breeze and the funneled breeze.

The "Columbia Gorge wind" is the only cold katabatic wind native to the United States. Residents of Portland, Oregon, are all too familiar with this strong wind and the cold spell it introduces. The origin of the wind is cold air that settles over the Columbia Plateau. As it sinks, the air follows the Columbia River Gorge westward through the Cascade Mountains. Upon reaching the coast, the cold Columbia Gorge wind replaces the mild coastal air.

The world's fiercest katabatic winds occur in Antarctica. There, the cold, dense air roars down the mountainsides and along the ice sheets constantly, for days and months on end. The wind averages 50 mph (80 kph) in many parts of the continent. It sometimes rages to 100 mph (160 kph), generating intolerable windchill equivalent temperatures below −100°F (−75°C).

Warm katabatic winds

Warm katabatic winds are generally set in motion by a larger-scale circulation pattern, for example the movement of strong upper-air westerlies across a mountain range. (Westerlies are global mid-latitude surface winds.) When a trough of low pressure is created over the mountain's leeward (opposite from the prevailing and local winds) slopes, it strengthens the high pressure system at the mountaintop and forces air down the mountains. Warm katabatic winds can also be drawn downhill by a strong cyclone (low-pressure system) or anticyclone (high-pressure system) located to the east of the mountains.

Chinook

The best known warm katabatic wind in North America is the chinook, a dry wind that blows down the eastern side of the Rocky Mountains, from New Mexico to Canada, in winter or early spring. Chinooks are also common on the eastern side of the Cascade Mountains in Washington.

Chinook winds originate as cool, dry air at the top of a high mountain. The air is dry because it has released most of its moisture, forming clouds, when it ascended the mountains' windward side, which is the side of the local or prevailing winds. As the air descends the leeward, or opposite side, slopes, it undergoes compressional warming at the dry adiabatic lapse rate. The dry adiabatic lapse rate is the rate that the temperature of a parcel of unsaturated air—air that has less than 100 percent humidity—changes as it ascends or descends. Specifically, the air warms by 5.5°F for every 1,000 feet (10°C per 300 meters) it descends. Given that air can travel 9,000 feet or more (5,000 meters or more) down the slopes of the Rockies, the warming may be considerable.

"Chinook" is an Arapaho Indian word meaning "snow eater." This name is appropriate for this wind because chinook winds bring a dramatic warming to winter-weary regions and melt snow in their path. Chinooks are dry; they may have less than 5 percent relative humidity, that is, 5 percent of the amount of water vapor possible relative to temperature, so they rapidly vaporize melted snow. A chinook can erase all signs of a foot-deep cover of snow in just a few hours.

Chinooks have been known to raise the temperature of an area by more than 35°F (20°C) in just one hour and by as much as 60°F (33°C) in a day. However, a chinook-induced warm spell does not mean that spring has come. The warm air can remain for several hours to days only to be displaced by cold winds from the west or the north. This can cause problems for plants that set buds or animals that start shedding winter coats too early.

While chinooks typically reach speeds of 25 to 50 mph (40 to 80 kph), they occasionally reach speeds greater than 100 mph (160 kph). The strongest chinook gust ever recorded was around 150 mph (230 kph) near Boulder, Colorado, on December 4, 1978. Violent chinooks can rip roofs off buildings and tear down trees and power lines. They whip up pebbles and debris, which can break windows and dent cars.

One warning sign of an approaching chinook is a chinook wall cloud. This solid bank of wispy, white clouds sometimes appears over the front range of the Rockies. A chinook wall cloud is formed as air rises along the windward slopes and moisture condenses. After this, air rapidly descends the leeward slopes.

Warm, dry winds similar to the chinook also occur in other parts of the world. Most notable is the foehn (or Fuhrn or fane), also spelled "föhn," that flows down from the Alps onto the plains of Austria and Germany. In Argentina this type of wind is called the zonda; in Romania it's called the austru; and in the Canterbury Plains of New Zealand it's called the nor'wester.

Weather report: Chinooks to remember

• On January 22, 1943, in Spearfish, South Dakota, the temperature rose 49°F (29°C) in just two minutes. At 7:30 am the thermometer read −4°F (−20°C) and at 7:32 am. It read 45°F (7°C)!
• On January 27, 1962, in Pincher Creek, Alberta, Canada, the temperature rose by 57°F (14°C) within one hour. It was −20°F (−29°C) at midnight and 37°F (3°C) at 1 am.
• On January 6, 1966, also in Pincher Creek, the temperature rose by 38°F (23°C) within four minutes.
• On January 7-8, 1969, outside of Boulder, Colorado, winds reached 130 mph (210 kph), with frequent gusts of more than 100 mph (160 kph). The greatest wind speed recorded within Boulder city limits was 97 mph (156 kph). That wind caused heavy property damage, including twenty-five roofs that were blown off.

Santa Ana winds

The Santa Ana winds are warm, dry, gusting katabatic winds from the east or northeast that create a major wildfire hazard in southern California. These winds occur between the months of October and February, peaking in intensity in December. In order to be classified as "Santa Anas" by the National Weather Service, the wind speed must be at least 30 mph (45 kph).

Santa Ana winds originate over the elevated plateau of the Mojave Desert and wind their way through the San Gabriel and San Bernardino mountains. They gain speed as they travel through the canyons and reach tremendous speeds in the Santa Ana Canyon, for which they are named. The winds then spill out into the foothills of the Los Angeles Basin and the San Fernando Valley.

The Santa Ana winds are generated by a high pressure system that sits above the Great Basin, the high-altitude plateau east of the Sierra Nevada Mountains and west of the Rockies. As this system turns clockwise, it pushes the air downward, over the edge of the high plateau, toward the lower pressure area at the coast.

Santa Ana winds blow with the force of about 40 mph (65 kph) on average, gusting to between 55 mph (90 kph) and 115 mph (185 kph). The strongest winds occur at night, in the absence of the sea breeze. The sea breeze blows in the opposite direction of the Santa Anas and acts as a counterforce.

As the air descends, it undergoes compressional warming in the same way as the chinooks. This air originates over the desert and, therefore, is dry from the start. Its relative humidity becomes even lower as it heats on descent. Santa Anas bring on heat waves throughout southern coastal California, with temperatures reaching 100°F (38°C) or higher.

As the Santa Ana winds travel across the dry, scrubby southern California vegetation, they further dry it out and turn it into perfect brush-fire fuel. The Santa Anas create conditions such that a single spark can set off a fire. Once the fire begins, the winds fan the flames into an inferno. The Santa Anas are also known for changing direction rapidly, which spreads fire to new areas.

Mud slides are a secondary problem brought about by the Santa Ana winds. Mud slides often occur after wildfire has removed the vegetation and left the slopes bare. If heavy winter rains fall before seeds have germinated and new vegetation has taken hold, the top layer of soil and debris will be washed away. Mud slides occasionally inundate roadways and even destroy homes.

Desert winds

Deserts are windy places, primarily because of surface heating. The temperature of dry ground on a sunny day may be exceedingly hot, in some places over 130°F (55°C). Air rises from the hot surface in a powerful convection, which starts surface winds blowing. Wind speeds are greatest during the hottest part of the day and during the hottest time of year.

In addition to transporting scorching heat from one place to another, desert winds may produce sandstorms. As strong winds blow across a desert, they lift up and carry along sand and dust. Sandstorms may take the form of billowing walls or clouds, or spinning whirlwinds.

Weather report: The losing battle against Mother Nature

Southern California's urban sprawl of recent decades has not stopped at the edge of wildfire- and mud slide-prone areas. Rather, homes have been and continue to be constructed in danger zones. Many foothill-dwellers have paid the price.

Since the start of southern California's building boom, Santa Ana wildfires and mud slides have caused billions of dollars in property damage and have claimed scores of lives. The following are some examples of Santa Ana-fueled disasters that have occurred in recent years:

• In October 1991, wildfire invaded Oakland and Berkeley. It burned down about 23,000 homes and killed 25 people.
• In October 1993, fifteen separate fires raged across the landscape from Ventura County to San Diego County. In all, the fires destroyed more than 1,200 buildings with damage totaling over $1 billion. One month later, fires fanned by 100 mph (160 kph) winds ravaged the outskirts of Malibu.
• In October 1996, fires in Malibu and Harmony Grove destroyed more than 150 homes and burned more than 41,000 acres.
• In 2003, California was beset by one of the worst wildfire seasons on record, prompting some to label it a "fire siege." Santa Ana-fueled fires raged across southern California, burned 750,000 acres, killed 24 people, and destroyed nearly 4,000 homes. Damage totaled $2 billion.

Sandstorms and dust storms

One type of sandstorm that occurs frequently in the deserts of the Sudan region of north-central Africa and occasionally in the southwestern United States is the haboob. This word is taken from the Arabic word habub, which means "blowing furiously." A haboob is a tumbling, black wall of sand that has been stirred up by cold downdrafts along the leading edge of a thunderstorm or a cold front. These downdrafts strike the hot, dusty ground and force the surface air, as well as the top layer of sand and dust, upward. The sand wall may rise a mile or more above the ground, sometimes all the way to the base of the thunderstorm cloud. Haboobs sometimes travel across distances greater than 90 miles (145 kilometers), reducing visibility to near zero.

A spinning vortex of sand and dust, called a dust devil, a whirlwind, or in Australia a willy-nilly, sometimes forms along the leading, cold air-warm air boundary of a haboob. More often, however, dust devils arise separately from haboobs, on clear, hot, relatively calm days. Fair-weather dust devils form over particularly warm areas, such as deserts, plowed fields, or flat expanses of dirt or pavement. Although dust devils bear a superficial resemblance to tornadoes, they form by different processes.

The first step in the formation of a dust devil is that hot air rises forcefully from the surface by convection, creating a low-pressure area at the surface. Next, surface winds converge to that point of low pressure. If there are horizontal layers of wind traveling at different speeds (a phenomenon called wind shear), rising air begins to spin around a vertical axis.

Dust devils are usually small and harmless, measuring less than 10 feet (3 meters) in diameter and less than 300 feet (90 meters) in height. They often last less than one minute. The largest dust devils reach a diameter of 100 feet (30 meters) and a height of 5,000 feet (1500 meters) and last for twenty minutes or more. The wind speed in dust devils may exceed 85 mph (135 kph).

While dust devils are generally harmless, they can sometimes cause significant damage, including overturning mobile homes and tearing roofs off buildings. A large and long-lived dust devil can toss over 50 tons (45 metric tons) of dust and debris into the sky.

On the shelves: Leaning on the Wind: Under the Spell of the Great Chinook

Published in 1995, this widely acclaimed collection of essays by Canadian writer Sid Marty, a former park ranger, explores the folklore and realities surrounding the chinook winds. Marty takes readers on astonishing adventures through the retelling of his life in southwest Alberta, Canada.

Sand and dust storms are also common occurrences in western Africa, due to the "harmattan" (pronounced har-ma-TAHN). The harmattan—also

WORDS TO KNOW

monsoon:

a name for shifting seasonal winds that result in a rainy season occurring in the summer on tropical continents, when the land becomes warmer than the sea beside it.

mountain breeze:

a gentle downhill wind that forms at night as cold, dense, surface air travels down a mountainside and sinks into the valley. Also called gravity wind or drainage wind.

nor'easter:

a strong northeasterly wind that brings cold air, often accompanied by heavy rain, snow, or sleet, to the coastal areas of New England and the mid-Atlantic states. Also called northeaster.

Northern Hemisphere:

the half of the Earth that lies north of the equator.

precipitation:

water in any form, such as rain, snow, ice pellets, or hail, that falls to Earth's surface.

pressure gradient:

the difference in air pressure between a high and low pressure area relative to the distance separating them.

radiational cooling:

the loss of heat from the ground upward into the atmosphere.

relative humidity:

the amount of water vapor in an air mass relative to the amount of water in a saturated air mass of the same temperature.

sea breeze:

the gentle wind that blows from over the sea to the shore during the day, due to differences in air pressure above each surface.

snow fence:

a device placed in fields and along highways that slows the wind and reduces the blowing and drifting of snow.

squall line:

a moving band of strong thunderstorms.

topography:

the shape and height of Earth's surface features.

trade winds:

an area of prevailing winds near the equator that blow from the northeast north of the equator and the southeast south of the equator.

transverse dune:

a series of connected barchan dunes, which appear as tall, elongated crescents of sand running perpendicular to the prevailing wind.

tsunami:

the largest type of water wave, generated by a submarine earthquake, landslide, or volcanic eruption.

unsaturated air:

air that has less than 100 percent relative humidity.

valley breeze:

an uphill wind that forms during the day as the valley air is heated and rises. Also called anabatic wind.

ventifact:

a rock, boulder, or canyon wall that has been sculpted by wind and wind-blown sand.

vertical cloud:

a cloud that develops upward to great heights. Vertical clouds are the products of sudden, forceful uplifts of small pockets of warm air.

westerlies:

global mid-latitude surface winds that travel from the southwest to the northeast in the Northern Hemisphere, and from the northwest to the southeast in the Southern Hemisphere, between about 30 and 60 degrees latitude.

wind shear:

a condition in which a vertical layer of air is sandwiched between two other vertical layers, each of which is traveling at a different speed and/or direction, causing the sandwiched air layer to roll.

windward:

the slope of a mountain on the side of local or prevailing winds up which cooler air ascends producing moist, cloudy, or rainy conditions.

spelled "harmatan," "harmetan," or "hermitan"—is a mild, dry, and dusty wind. It is an easterly or northeasterly wind that originates over the Sahara during the cool winter months, from late November through mid-March. The harmattan blows across the continent to Africa's west coast, where, despite its dryness, it brings a welcome relief from the intense tropical heat and humidity.

The negative side of a harmattan is that it can create towering sand and dust storms, up to 20,000 feet (6,100 meters) high. Over 100 million tons (90 million metric tons) of dust are deposited into the Atlantic Ocean annually by harmattan dust storms.

Winds of the Sahara

In most cases, the winds that originate in the Sahara Desert in northern Africa are northerly winds, meaning they blow to the south. However, the presence of storm systems at certain locations may redirect these winds, turning them into southerly winds. In such cases, the winds blow across the Mediterranean Sea and into southern Europe or the Middle East. These patterns usually occur in spring or fall.

There are several names for the winds of the Sahara, depending on their point of origin and their destination.

The "leste" (pronounced LESS-tay) is a hot, dry wind that comes from Morocco or Algeria. When a storm system is present off the northwest tip of Africa, just southwest of Spain, this wind blows out over the Atlantic Ocean or the Mediterranean Sea. If the leste crosses the Mediterranean and blows onto southern Spanish shores, it is called the "leveche" (pronounced luh-VAY-chay). The leveche, like the leste, is hot and dry. The wind picks up only a very small amount of moisture during its short trip across the water.

The sirocco (pronounced suh-ROCK-oh), in contrast, has a longer journey across the Mediterranean. Hence, by the time this dry, dusty southeasterly wind out of North Africa reaches Sicily and southern Italy, it has become warm and humid. The sirocco is generated when a storm system is positioned to the southwest of Italy, over the Mediterranean.

The word "sirocco" (often spelled "scirocco") comes from the Arabic word suruk, which means "rising of the sun." In the central Sahara region, this wind is called "shahali" (pronounced SHA-ha-lee); in Tunisia it is called "chili" (pronounced SHILL-ee); and in southern Algeria it is called "chichili" (pronounced CHEE-chi-lee).

Another hot, dry, southerly wind originating on the Sahara, this one in Libya and Egypt, is the "khamsin" (pronounced kahm-SENE). When a storm is present to the north-northeast, over Turkey, the khamsin blows over the northern tip of the Red Sea and into Saudi Arabia, Jordan, and Israel. The khamsin is a strong wind that produces large sand and dust storms. The name for this wind in Israel is the "sharav" (pronounced shahr-AHV).

The khamsin reappears regularly each year. Its name is the Arabic word for "fifty" because it blows for about fifty days continuously, starting in mid-March. The air carried by the khamsin has a temperature greater than 120°F (50°C) and a relative humidity of 10 percent.

The "simoom" (pronounced si-MOOM) is a dry, blustery, dust-laden wind that blows across the Sahara and the deserts of Israel, Syria, and the Arabian peninsula. It often reaches temperatures of more than 130°F (55°C), with a relative humidity less than 10 percent. The simoom, which can cause the life-threatening condition heat stroke, is nicknamed the "poison wind." The word "simoom" comes from the Arabic word semum, which means "poisoning."

"Gharbi" (pronounced GAHR-bee) is the name of a wind that originates over the Atlantic Ocean, sweeps across Morocco, and travels over the Mediterranean. This wind picks up dust as it crosses the desert and moisture as it crosses the water. It then deposits heavy rains on the lands of the north and east Mediterranean region. Due to the sand and dust, this precipitation is reddish and is called "red rain."

Other desert winds

The "berg" wind originates in the interior of South Africa. It blows down the mountains and out to the coast. This wind is dry, dusty, and very hot. "Berg" is an Afrikaans word meaning "mountains."

The "brick fielder" is Australia's version of a dry, dusty, very hot wind. It comes from the central desert region in the summer months and blows heat, dust, and sand toward the southeastern coast.

The "shamal" (pronounced shah-MALL) is a northwesterly wind that blows throughout the Persian Gulf and the lower valley of the Tigris and Euphrates rivers in Iraq. This hot, dry, and dusty wind can begin to blow suddenly, at any time of year. It typically blows for forty days continuously in June and early July, in what is known as the "great shamal" or the "forty-day shamal." At all other times of year, it generally lasts from one to five days and becomes calm at night.

"Shamal" is the Arabic word for "left-hand" or "north." Another name for this wind is "barih" (pronounced BAR-ee).

Cold winds

One of the more familiar examples of a regional cold wind is the "Texas norther." As its name implies, a Texas norther is a northerly wind that dips far south, bringing cold air into Texas. Texas northers (sometimes just called "northers") usually follow in the wake of an intense winter storm traveling eastward across the United States. A Texas norther can lower the temperature in Texas by dozens of degrees in just a few hours.

A Texas norther that causes a dramatic drop in temperature, possibly accompanied by sleet and snow, is called a "blue norther." A Texas norther that continues southward and brings cold air into Central America is called "el norte."

A nor'easter or "northeaster" is a strong, northeasterly wind that brings cold air to the coastal areas of New England and the mid-Atlantic states, occasionally as far south as Florida. Nor'easters are generated by extratropical cyclones in the Atlantic (cyclones that develop at high latitudes). These storms develop or intensify off the eastern seaboard of North America and move to the northeast along the coast. The gale-force wind that spins off the storm is often accompanied by heavy rain, snow, or sleet.

One of the most extreme cold winds is Australia's "southerly buster" (originally and properly called "southerly burster"). This violent, cold wind, which comes from the south, represents the leading edge of a strong cold front. A southerly buster can lower the temperature in southeastern Australia by as much as 36°F (20°C) in just a few minutes.

The strong cold wind of central Asia is called the "buran" (pronounced boo-RAN). This dreaded wind originates in Siberia and brings unbearably cold blasts into Russia and the central Asian republics. When the buran is accompanied by snow, which may be heavy, it is called "purga" (pronounced POOR-guh).

Weather report: Daunting dust devils

One of the worst dust devils on record occurred in May 1995 near Minong, Wisconsin. It left a 300-yard-long (275-meter-long) path of destruction, including a damaged roof, a torn-up snow fence, and a downed power line that started a fire.

Another destructive dust devil rose up completely unannounced on a sunny day in March 1995 in upstate South Carolina. The whirlwind smashed the covered porch of a home and carried the wreckage hundreds of yards (meters) away.

In the spring of 1991, a dust devil passed right by the National Weather Service station at Albuquerque, New Mexico. The station's anemometer, a device that measures windspeed, measured the wind gusts at 70 mph (110 kph).

A cold, snowy wind similar to Russia's purga is Alaska's "burga" (pronounced BOOR-guh), also spelled "boorga." This wind comes from the northeast and may carry sleet as well as snow.

A cold, dry wind that comes from the north or northeast and invades southern Europe is called a "bise" (pronounced BEEZ, also spelled "bize"). This wind blows in the winter and early spring. It sometimes brings on frosts after the start of the growing season, thus endangering crops.

A "tehuantepecer" (pronounced te-WAHN-te-peck-er) is a cold, blustery, northerly wind that blows down from the Gulf of Mexico. This winter wind picks up speed as it crosses the mountains between Mexico and Guatemala. It then spills out into the Gulf of Tehuantepec, on the southern coast of Mexico, and can blow for 100 miles (160 kilometers) over the sea.

The "pampero" (pronounced pahm-PAIR-oh) is a South American wind. Similar to the Texas norther, the pampero descends from the plains and brings bitterly cold air to typically warm regions. This southwesterly wind originates in the Andes Mountains and blows across the grasslands (pampas, in Spanish) of Argentina and Uruguay and out to the Atlantic Coast of Brazil. The pampero is often accompanied by thunderstorms and brings about a rapid drop in temperature.

The coldest local wind of all is the "whirly." The whirly, sometimes classified as a storm, is a small, violent squall in Antarctica. It is a rapidly spinning wind that whips up snow across an area ranging from a few yards to hundreds of yards in diameter. This windstorm usually occurs during the transition periods between the light Antarctic summer and dark Antarctic winter.

Other local winds

There are several other kinds of local winds that do not fall neatly into any of the categories listed above. They have been grouped together in this section.

Weather extremes: The Great Blue Norther of 11/11/11

On November 11, 1911, a weather event widely known as The Great Blue Norther of 11/11/11 produced the most dramatic temperature changes in U.S. history. The temperature in several places plunged over 60°F (30°C) in a few hours. By nightfall, several cities were dealing with single-digit temperatures. Kansas City, Missouri, had just established a record high temperature for the date of 76°F (25°C) in the late morning. By midnight, the temperature had dropped to 11°F (−12°C), setting a record low temperature for the same date.

Several cities broke high and low records that day. Springfield, Missouri, had set a record high of 80°F (27°C). Within two hours, the temperature had dropped to 40°F (4°C) and before midnight the temperature had plummeted to a record 13°F (−11°C). Oklahoma City also set record high and low temperatures on the same date, with a high of 83°F (28°C) and a low of 17°F (−8°C).

Derechos

A derecho (pronounced day-RAY-cho) is a destructive, hurricane-force wind that travels in a straight line. This wind was named in 1883 by the director of the Iowa Weather Service, Gustavus Hinrichs. Hinrichs decided on the name "derecho," a Spanish word meaning "straight ahead," to underscore the difference between this wind and a tornado, which spins. "Tornado" is from the Spanish word tornar meaning "to return" or "to change."

To be defined as a derecho, the winds must travel faster than 58 mph (93 kph) and the path of the damage must be at least 280 miles (450 kilometers) long. Derecho winds sometimes exceed 100 mph (160 kph) and cause damage across a 150-mile-wide (240-kilometer-wide) area. Derechos are relatively long-lived. Some have swept across several states in a period of sixteen hours.

Weather extremes: Ravaging nor'easters

In February 1969, a storm with nor'easter winds dumped record-setting snowfalls throughout New England. In Rumford, Maine, snow accumulation totaled 70 inches (175 centimeters). There were 114 inches (290 centimeters) of snow in Mansfield, in the mountains of Vermont; and 164 inches (415 centimeters) at Pinkham Notch, in the mountains of New Hampshire.

In March 1984, a storm struck the Atlantic coast from Virginia to Maine. The shoreline was battered by waves that rose to heights of 20 feet (6 meters), along with 92 mph (148 kph) nor'-easter winds. This storm destroyed beaches, homes, boardwalks, and seawalls. The damage totaled over $1 billion.

A derecho is caused by a series of intense downbursts, which are extremely strong, localized downdrafts, from a squall line, or cluster of thunderstorms. These thunderstorms collectively cover an area that is 75 to 300 miles (120 to 480 kilometers) wide. They act as a single storm with tremendous force. The downbursts

of a derecho form when a line of thunderstorms passes over a layer of dry air that is several thousand feet above ground. As precipitation, or water in any form, falls into this dry air, moisture rapidly evaporates, thus cooling the air. The cool, heavy air that is formed, assisted by the downward motion of the raindrops, then surges toward the ground. These forceful downbursts together form a derecho. They often spawn tornadoes as well.

Monsoons

A "monsoon" is a seasonal wind that occurs throughout southeast Asia, along the Atlantic coastal regions of northern South America, and on the coasts of central Africa. The name "monsoon" comes from the Arabic word mausim, meaning "season." This wind blows throughout the summer months, bringing heavy rains and flooding to a region that is dry for most of the winter.

Weather extremes: Destructive derechos

Derechos occur most often at night, in the late spring and summer. They usually occur in the central and northern Great Plains states and the Midwest, and sometimes as far east as New York.

In July 1995, a derecho blasted through New York State and southern New England in just three and a half hours. It ravaged rural upstate New York with wind gusts of up to 106 mph (170 kph), and was accompanied by thousands of lightning strikes, each of which is produced by a 100 million-volt electrical discharge. The wind storm knocked down tens of millions of trees across the nearly one-million-acre (four-hundred-thousand-hectare) Adirondack State Park, piling felled trees 10 to 20 feet (3 to 6 meters) high. Four people who were camping in the park were killed by the falling trees.

In 1999, an unusually long-lived derecho hit the border area between the United States and Canada. The so-called "Boundary Waters-Canadian Derecho" began on July 4 and lasted twenty-two hours, with winds gusting above 100 mph (160 kph). The system traveled 1,300 miles (2,092 kilometers) from northen North Dakota to New England, injuring dozens, killing two, and causing property damage in excess of $100 million.

In June 2004, a derecho tore through northern Louisiana, southern Arkansas, east Texas, and Oklahoma, killing one person, uprooting trees, and overturning cars and trucks.

St. Louis, Missouri, was hit by two derechos in three days: one on July 21 and one on July 23, 2006. The winds, topping 80 mph (129 kph), knocked out power for more than 600,000 people and cut a swath of damage 400 miles long and several miles wide.

Throughout the winter, the region affected by monsoons is tilted away from the sun. As a result, the sea is warmer than the land during that time. A land breeze forms, and hot, dry winds from far inland are blown out toward the sea.

When spring arrives, the monsoon area moves to a position almost directly beneath the sun and the winds shift direction. The land is heated intensely, which causes surface air to rise. This air is replaced by moist winds from over the ocean, similar to a sea breeze. The moisture from these winds forms clouds which yield heavy rains.

Levanters

The "levanter" (pronounced li-VAN-ter, also spelled "levante") is the most pleasant wind in the Mediterranean region. The levanter is a fresh, mild easterly or northeasterly wind that blows across the southern coast of France, the eastern coast of Spain, and through the Straits of Gibraltar. It is named for Levant, a region along eastern shores of the Mediterranean Sea.

The levanter travels over the Mediterranean, which makes it humid. This often strong wind brings overcast skies and rain. It occurs most frequently in June through October.

Konas

Hawaii is famous for its overall pleasant weather, which is largely influenced by the trade winds, which are the prevailing winds near the equator. The "kona" (pronounced KOH-nuh) winds, which usher in heavy rains and storms, stand in contrast to this trend. The konas are southwesterly winds that blow down leeward (the Hawaiian word for which is "kona") slopes of Hawaii's mountains, about five times each winter.

These warm, very humid winds are of moderate strength. They may produce intense storms with heavy rainfall or steady to light rainfall, lasting from several hours to several days.

The wind's effect on surfaces

Local winds are like sculptors, etching away at surfaces and objects standing in their path. The winds blow the sand, snow, and water, and even shape rock walls and trees. The effect of the winds can be gradual and long term, such as where it shapes the buttes of the American West and the sand dunes of the Sahara Desert. The effect of the winds can also be much more immediate and short term, such as where it whips water into waves and blows snow into drifts.

Sand formations

Even a moderate desert wind can set sand in motion. A wind of about 15 mph (24 kph) is strong enough to move very small grains of sand, with diameters of about .008 inches (.2 millimeters). At speeds of 30 mph (48 kph), the wind can move grains with diameters of about .08 inches (2 millimeters). However, the tiniest particles of dust and sand can not be moved directly by the wind. The reason is that a very shallow layer of calm air, extending only about .004 inches (.1 millimeters) above the ground, is unaffected by the wind. The tiny particles are stirred only when they are struck by moving particles.

Weather report: Hot winds and human health

Numerous local winds are thought to be responsible for temporary declines in the mental and physical health of a region's inhabitants. While the health-wind connections in some cases are popularly accepted as fact, it remains a mystery as to whether or not scientific processes are involved. What follows are examples of winds and the alleged negative effects they have on human health.

• The chinooks are reported to cause irritability, depression, and illness.
• The sirocco winds are claimed to cause fatigue and mental weakness.
• The Santa Ana winds seem to make people nervous, anxious, and possibly even homicidal.
• The foehns are reported to increase the frequency of suicide attempts among people who have suicidal thoughts.

The statistical links between these various irritating winds and human psychological problems are weak and the effect, if any, is small. The frequency of suicide during a foehn event might increase by less than 10 percent. However, the alleged effect has been given the name "Föhnkrankheit" or "Föhn-sickness", and researchers are searching for a cause-and-effect relationship.

When the wind blows, it sets in motion a process called saltation, the migration of particles along the ground and through the air. When sand particles are set in motion by the wind, they first slide along the surface. Once these particles overtake and strike other particles, some of the particles bounce into the air and are carried along by the wind. These particles, in turn, fall back to the ground and kick other particles up, into the wind. Eventually, through this process, sand can be blown across great distances.

As sand is continually carried away from a particular location, the surface level becomes lower and lower. Eventually, if all the sand and dust is removed, all that remains is hard, flat, dry ground and gravel. This type of surface is called desert pavement. The bare, dry floor of the Gobi Desert in Mongolia is an example of desert pavement. Desert pavements can also be formed outside of deserts. Agricultural fields, for instance, may turn to desert pavement when soil erosion is accompanied by prolonged drought.

The wind may also sculpt the sand into sand dunes. Sand dunes form when billions of sand grains accumulate in a given location. A sand dune is a mound of sand that is produced over time by a strong wind blowing in a fairly constant direction. Blowing sand comes to a halt behind obstacles, such as rocks or plants. When an accumulation of sand becomes large enough, the sand pile itself acts as an obstacle behind which blowing sand continues to gather.

The angle of a sand dune is more gradual on the windward side, where the wind blows strongest, and steeper on the leeward side, where the wind is calm by comparison. The difference in angle of incline from one side of the dune to the other results because the wind blows the sand up the windward face to the top of the dune. On reaching the top of the dune, the sand merely drops to the other side. By examining the shape of a sand dune, it is possible to tell the direction of the prevailing winds at the time the dune was formed.

Entire dunes are also nudged along by the wind. Sand dunes move as much as 50 feet (15 meters) a year.

Barchan (pronounced bar-KHAN) dunes, also called barchane, barkhan, or crescentic dunes, are formed by winds blowing in a nearly constant direction and moderate speed, across relatively flat land with only a shallow layer of sand. When viewed from above, these dunes resemble crescent moons, with the tips of the crescent pointing downwind.

Weather extremes: The windiest places in the world

Declaring a particular spot the windiest place is a bit tricky. For one thing, recorded wind speeds are probably not the highest speed winds that have ever happened on Earth. Faster winds have probably occurred at other locations, but were not recorded. The windiest spot on Earth is even more difficult to establish because the place with the highest average wind speed is not necessarily the place with the highest recorded wind speed.

Commonwealth Bay, Antarctica, is home to the world's highest recorded average wind speeds, so it is considered by many to be the world's windiest place. It was so named in 1912 by an early Australian explorer, Douglas Mawson (1882–1958), who spent two winters there. This bay is situated on the coast of Eastern Adelie Land and West King George Land. It is on the Indian Ocean side of the continent, opposite Australia.

The wind at Commonwealth Bay was measured in 1951 by a team of French scientists who established a base there. They clocked the cold katabatic winds at 40 mph (65 kph) on average, for the year. The highest monthly average winds were 65 mph (105 kph), in March. The highest average winds in a twenty-four-hour period were 108 mph (174 kph), on March 21-22. The winds regularly gusted to over 200 mph (320 kph).

Another candidate for windiest place on Earth, and certainly the windiest place in the United States, is Mount Washington, New Hampshire. The Mount Washington Observatory, located at an altitude of 6,288 feet (1,915 meters), recorded a wind gust of 231 mph (371 kph) on April 12, 1934. Subsequently, there have been frequent gusts up to 220 mph (355 kph) recorded there. Mount Washington also holds the U.S. record for the highest average wind speed in a twenty-four-hour period. On April 12, 1932, the wind blew at an average speed of 130 mph (210 kph).

Where sand is more plentiful, a series of connected barchan dunes may form. This structure, called transverse dunes, appears as tall, elongated crescents of sand running perpendicular to the prevailing winds.

Barchan dunes with very pronounced crescent shapes, that exist either singly or in connected lines, are called seif (pronounced safe) dunes. These dunes are very steep; the crest of a seif dune forms a sharp ridge.

"Seif" is an Arabic word meaning "sword." These dunes are so named because their shape resembles the curved blade of a sword. The seif dunes of Algeria and Iran reach up to 650 feet (197 meters) in height. When seifs exist in a series, they form a meandering pattern that is caused by shifting winds.

Sand ripples are wavy designs formed by the motion of sand along the surface of a sand dune. The ripples run in a direction perpendicular to the wind. Like the sand dune, sand ripples have a more gradual incline on the windward side of the dune and a steeper incline on the leeward side. The direction of the sand ripples changes whenever the wind changes direction.

Sculpted objects

Another consequence of wind and wind-blown sand is the eroding and shaping of solid objects. Rocks that have been sculpted by these forces are called ventifacts. Ventifacts include not only rocks lying on the ground, but larger structures such as boulders and canyon walls. Surface rocks, however, constitute the bulk of ventifacts. The reason for this fact is that wind-borne sand generally travels close to the ground.

Wind and blowing sand can also erode the bases of solid rock structures, such as boulders, canyons, and cliffs. The wind has the greatest effect on soft rock, such as sandstone. The windward side of a boulder may be rough and pitted, even notched, while its leeward side is relatively smooth. Sometimes wind-blown sand has the effect of polishing rocks. This effect occurs most often in areas prone to sandstorms, where billions of sand grains batter the rocky surfaces.

Ventifacts abound in Utah's Goblin Valley. The sandstone formations there, made of alternating hard and soft layers, have been sculpted into strange and beautiful shapes by the wind. One particular type of ventifact found in Goblin Valley is the "mushroom rock." Mushroom rocks form from boulders, the lower portions of which are made of soft sandstone and the upper portions of which are made of hard sandstone. The wind erodes the soft portion at a much faster rate than the hard portion. Thus, these rocks look like mushrooms, with large heads of hard sandstone sitting atop skinny stems of soft sandstone.

One of the world's largest and most magnificent collections of ventifacts is found in Bryce Canyon, Utah. There, the wind (as well as water) has battered the tops of tall, layered rock formations. The erosion of the outer layers has created a breathtaking array of stone spires.

Rock is not the only material to be shaped by wind-blown sand. Telephone poles, trees, and cars can also be sculpted in the desert. Because of the constant beating by wind-blown sand on lower portions of telephone poles, the poles become narrower at the base than they are higher up.

Snow formations

Another medium sculpted by the wind is snow. Snow can be blown into large drifts called snow dunes that are similar to sand dunes. Snow dunes form when a strong wind comes along after snow has fallen on a flat landscape. The wind carries the snow until it meets a barrier, where it deposits the snow. In the Great Plains states, when snow falls on fields and is blown away, the first obstacles it meets are typically in populated areas. For this reason, snow accumulation may be negligible in the countryside at the same time that there are several inches (centimeters) in town.

Gentler winds form ripples in the snow, rather than blowing it away. Snow ripples, similar to sand ripples, are long wavelike patterns that run perpendicular to the direction of the wind.

Snow ripples in Antarctica and other very cold places are called sastrugi (pronounced SASS-truh-ghee). These patterns form when the wind blows from the same direction for several days. Sastrugi freeze solid and can remain for a long time. The singular form of "sastrugi" is "sastruga," which comes from the Russian word zastruga, meaning "wind-made furrow."

Water formations

In addition to sand and snow, the surface of water is also shaped by the winds. However, only one pattern can be created on the water's surface: waves. Wind-driven waves are technically known as wind waves. While the wind is the most common cause of surface waves, it is not the only cause. Some other forces that give rise to waves include: tides, volcanic activity, and earthquakes beneath the ocean floor.

Did you know? Snow fences

Snow fences are devices that slow down winds and reduce the blowing and drifting of snow. They are erected in two areas: in fields and along highways. The purpose of snow fences in fields is to prevent the wind from stripping the ground of snow. A blanket of snow protects the soil by insulating it against bitter cold temperatures. The snow also provides much needed moisture when it melts.

The purpose of snow fences along a highway is to reduce the amount of snow that blows across or piles up on the road. As the wind whips across a clearing, it picks up snow. When this snow-laden wind hits a snow fence, its speed is reduced. A slower wind will deposit its snow, and at the same time will pick up very little new snow. The snow that's deposited forms a gradual drift on the other (downwind) side of the snow fence.

A snow fence must be placed far enough from the road so that most of the snow gets deposited between the fence and the road, with as little snow as possible reaching the road. The rule of thumb is that snow will accumulate downwind of the fence for a distance approximately ten times the height of the fence. Thus, if a fence is 10 feet (3 meters) high, it should be placed at least 100 feet (30 meters) from the road.

Wave height is directly proportional to wind speed. To calculate the height of waves precisely, however, requires knowledge of the length of time the wind has been blowing, as well as the distance over water, or fetch, the wind is blowing. The longer the time and distance over which the wind blows, the taller the waves will be.

It stands to reason that the largest wind waves are generated by large, stationary storm systems. The tallest wind wave ever recorded was 112 feet (34 meters) high. This wave occurred during a storm on the Pacific Ocean, with winds of nearly 70 mph (112 kph), on February 7, 1933. This wave was not the tallest of all waves, however. That title is reserved for another class of waves called tsunamis, which are generated by submarine earthquakes.

[See AlsoClimate; Monsoon; Tsunami; Weather: An Introduction ]

For More Information

BOOKS

Simpson, John E. Sea Breeze and Local Winds. New York: Cambridge University Press, 1994.

Van Sant, Bruce. Tricks of the Trades. St. Petersburg FL: Cruising Guide Publications, Inc., 2001.

Walker, Stuart H. Sailor's Wind. New York: W. W. Norton, 2006.

Watts, Allan. Instant Wind Forecasting. New York: Sheridan House, 2002.

WEB SITES

"Forces and Winds: Online Meteorology Guide." University of Illinois: Weather World 2010 Project. 〈http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/fw/home.rxml〉 (accessed March 22, 2007).

"Local Winds: Mountain Breezes." Danish Wind Industry Association. 〈http://www.windpower.org/en/tour/wres/mount.htm〉 (accessed March 22, 2007).

"Local Winds: Sea Breezes." Danish Wind Industry Association. 〈http://www.windpower.org/en/tour/wres/localwin.htm〉 (accessed March 22, 2007).

"Names of Winds." Golden Gate Weather Services. 〈http://ggweather.com/winds.html〉 (accessed March 22, 2007).