Wind

2007 Schools Wikipedia Selection. Related subjects: Climate and the Weather

Wind is the roughly horizontal movement of air (as opposed to an air current) caused by uneven heating of the Earth's surface. It occurs at all scales, from local breezes generated by heating of land surfaces and lasting tens of minutes to global winds resulting from solar heating of the Earth. The two major influences on the atmospheric circulation are the differential heating between the equator and the poles, and the rotation of the planet ( Coriolis effect).

Given a difference in barometric pressure between two air masses, a wind will arise between the two which tends to flow from the area of high pressure to the area of low pressure until the two air masses are at the same pressure, although these flows will be modified by the Coriolis effect in the extratropics.

Winds can be classified either by their scale, the kinds of forces which cause them (according to the atmospheric equations of motion), or the geographic regions in which they exist. There are global winds, such as the wind belts which exist between the atmospheric circulation cells. There are upper-level winds, such as the jet streams. There are synoptic-scale winds that result from pressure differences in surface air masses in the middle latitudes, and there are winds that come about as a consequence of geographic features such as the sea breeze. Mesoscale winds are those which act on a local scale, such as gust fronts. At the smallest scale are the microscale winds which blow on a scale of only tens to hundreds of metres and are essentially unpredictable, such as dust devils and microbursts.

Winds can also shape landforms, via a variety of eolian processes.

Winds by spatial scale

Prevailing winds — the general circulation of the atmosphere

Prevailing winds are winds which come about as a consequence of global circulation patterns. These include the Trade Winds, the Westerlies, the Polar Easterlies, and the jet streams.

Because of differential heating and the fact that warm air rises and cool air falls, there arise circulations that (on a non-rotating planet) would lead to an equator-to-pole flow in the upper atmosphere and a pole-to-equator flow at lower levels. Because of the Earth's rotation, this simple situation is vastly modified in the real atmosphere. In almost all circumstances the horizontal component of the wind is much larger than the vertical — the exception being violent convection.

The Trade Winds are the most familiar consistent and reliable winds on the planet, exceeded in constancy only by the katabatic winds of the major ice sheets of Antarctica and Greenland. It was these winds that early mariners relied upon to propel their ships from Europe to North and South America. Their name derives from the Middle High German trade, akin to Old English trod meaning "path" or "track", and thus the phrase "the wind blows trade", that is to say, on track.

The Trades form under the Hadley circulation cell, and are part of the return flow for this cell. The Hadley carries air aloft at the equator and transports it poleward north and south. At about 30°N/S latitude, the air cools and descends. It then begins its journey back to the equator, but with a noticeably westward shift as a result of the action of the Coriolis force.

Along the east coast of North America, friction twists the flow of the Trades even further clockwise. The result is that the Trades feed into the Westerlies, and thus provide a continuous zone of wind for ships travelling between Europe and the Americas.

The Westerlies, which can be found at the mid-latitudes beneath the Ferrel circulation cell, likewise arise from the tendency of winds to move in a curved path on a rotating planet. Together with the airflow in the Ferrel cell, poleward at ground level and tending to equatorward aloft (though not clearly defined, particularly in the winter), this predisposes the formation of eddy currents which maintain a more-or-less continuous flow of westerly air. The upper-level polar jet stream assists by providing a path of least resistance under which low pressure areas may travel.

The Polar Easterlies result from the outflow of the Polar high, a permanent body of descending cold air which makes up the poleward end of the Polar circulation cell. These winds, though persistent, are not deep. However, they are cool and strong, and can combine with warm, moist Gulf Stream air transported northward by weather systems to produce violent thunderstorms and tornadoes as far as 60°N on the North American continent.

Records of tornadoes in northerly latitudes are spotty and incomplete because of the vast amount of uninhabited terrain and lack of monitoring, and it is certain that tornadoes have gone unseen and unreported. The deadly Edmonton tornado of 1987, which ranked as an F4 on the Fujita scale and killed 27 people, is evidence that powerful tornadoes can occur north of the 50th parallel.

The Edmonton, Canada tornado of 1987 is evidence that powerful tornadoes can develop at high latitudes.

The jet streams are rapidly moving upper-level currents. Travelling generally eastward in the tropopause, the polar jets reside at the juncture of the Ferrel cell and the Polar cell and mark the location of the polar cold front. During winter, a second jet stream forms at about the 30th parallel, at the interface of the Hadley and Ferrel cells, as a result of the contrast in temperature between tropical air and continental polar air.

The jet streams are not continuous, and fade in and out along their paths as they speed up and slow down. Though they move generally eastward, they may range significantly north and south. The polar jet stream also marks the presence of Rossby waves, long-scale (4000 - 6000 km in wavelength) harmonic waves which perpetuate around the globe.

Seasonal winds

Seasonal winds are winds that only exist during specific seasons, for example, the Indian monsoon.

Synoptic winds

Synoptic winds are winds associated with large-scale events such as warm and cold fronts, and are part of what makes up everyday weather. These include the geostrophic wind, the gradient wind, and the cyclostrophic wind.

As a result of the Coriolis force, winds in the northern hemisphere always flow clockwise (when seen from above) around a high pressure area and counterclockwise around a low pressure area (the reverse occurs in the southern hemisphere). At the same time, winds always flow from areas of high pressure to areas of low pressure. These two forces are opposite but not equal, and the path that results when the two forces cancel each other runs parallel to the isobars. Wind following this path is known as geostrophic wind. Winds are said to be truly geostrophic only when other forces (e.g. friction) acting on the air are negligible, a situation which is often a good approximation to the large-scale flow away from the tropics.

In certain circumstances, the Coriolis force acting on moving air may be almost or entirely overwhelmed by the centripetal force. Such a wind is said to be cyclostrophic, and is characterized by rapid rotation over a relatively small area. Hurricanes, tornadoes, and typhoons are examples of this type of wind.

Mesoscale winds

Synoptic winds occupy the lower boundary of what is considered to be "forecastable" wind. Winds at the next lowest level of magnitude typically arise and fade over time periods too short and over geographic regions too narrow to predict with any long-range accuracy. These mesoscale winds include such phenomena as the cold wind outflow from thunderstorms. This wind frequently advances ahead of more intense thunderstorms and may be sufficiently energetic to generate local weather of its own. Many of the "special" winds, addressed in the last section of this article, are mesoscale winds.

Microscale winds

Microscale winds take place over very short durations of time - seconds to minutes - and spatially over only tens to hundreds of metres. The turbulence following the passage of an active front is composed of microscale winds, and it is microscale wind which produces convective events such as dust devils. Though small in scope, microscale winds can play a major role in human affairs. It was the crash of a fully loaded Lockheed L-1011 at Dallas-Fort Worth International Airport in the summer of 1985, and the subsequent loss of 133 lives, that introduced the term "microburst" to many people, and that was a factor in the installation of doppler radar in airports and weather installations worldwide.

Winds by effect

In classical terminology, Aeolian winds, or winds producing Aeolian action, are winds which produce geologic changes. Modern tornadoes and hurricanes might at times be considered to produce such changes.

Largescale erosion, dune formation, and other geologic and topographic effects influenced by wind are still referred to as aeolian activity.

Local winds that are tied to specific temperature distributions

Some local winds blow only under certain circumstances, i.e. they require a certain temperature distribution.

Differential heating is the motive force behind land breezes and sea breezes (or, in the case of larger lakes, lake breezes), also known as on- or off-shore winds. Land is a rapid absorber/radiator of heat, whereas water absorbs heat more slowly but also releases it over a greater period of time. The result is that, in locations where sea and land meet, heat absorbed over the day will be radiated more quickly by the land at night, cooling the air. Over the sea, heat is still being released into the air at night, which rises. This convective motion draws the cool land air in to replace the rising air, resulting in a land breeze in the late night and early morning. During the day, the roles are reversed. Warm air over the land rises, pulling cool air in from the sea to replace it, giving a sea breeze during the afternoon and evening.

Mountain breezes and valley breezes are due to a combination of differential heating and geometry. When the sun rises, it is the tops of the mountain peaks which receive first light, and as the day progresses, the mountain slopes take on a greater heat load than the valleys. This results in a temperature inequity between the two, and as warm air rises off the slopes, cool air moves up out of the valleys to replace it. This upslope wind is called a valley breeze. The opposite effect takes place in the afternoon, as the valley radiates heat. The peaks, long since cooled, transport air into the valley in a process that is partly gravitational and partly convective and is called a mountain breeze.

Mountain breezes are one example of what is known more generally as a katabatic wind. These are winds driven by cold air flowing down a slope, and occur on the largest scale in Greenland and Antarctica. Most often, this term refers to winds which form when air which has cooled over a high, cold plateau is set in motion and descends under the influence of gravity. Winds of this type are common in regions of Mongolia and in glaciated locations.

Because katabatic refers specifically to the vertical motion of the wind, this group also includes winds which form on the lee side of mountains, and heat as a consequence of compression. Such winds may undergo a temperature increase of 20 °C (36 °F) or more, and many of the world's "named" winds (see list below) belong to this group. Among the most well-known of these winds are the chinook of Western Canada and the American Northwest, the Swiss föhn, California's infamous Santa Ana wind, and the French Mistral.

The opposite of a katabatic wind is an anabatic wind, or an upward-moving wind. The above-described valley breeze is an anabatic wind.

A widely-used term, though one not formally recognised by meteorologists, is orographic wind. This refers to air which undergoes orographic lifting. Most often, this is in the context of winds such as the chinook or the föhn, which undergo lifting by mountain ranges before descending and warming on the lee side.

Winds that are defined by an equilibrium of physical forces

These winds are used in the decomposition and analysis of wind profiles. They are useful for simplifying the atmospheric equations of motion and for making qualitative arguments about the horizontal and vertical distribution of winds. Examples are:

• Geostrophic wind (wind that is a result of the balance between Coriolis force and pressure gradient force; flows parallel to isobars and approximates the flow above the atmospheric boundary layer in the midlatitudes if frictional effects are low)
• Thermal wind (not actually a wind but a wind difference between two levels; only exists in an atmosphere with horizontal temperature gradients, i.e. baroclinicity)
• Ageostropic wind (difference between actual and geostrophic wind; the wind component which is responsible for air "filling up" cyclones over time)
• Gradient wind (like geostrophic wind but also including centrifugal force)

Names for specific winds in certain regions

In ancient Greek mythology, the four winds were personified as gods, called the Anemoi. These included Boreas, Notos, Euros, and Zephyros. The Ancient Greeks also observed the seasonal change of the winds, as evidenced by the Tower of the Winds in Athens.

In modern usage, many local wind systems have their own names. For example:

• Abroholos (squall frequent wind that occurs from May through August between Cabo de Sao Tome and Cabo Frio on the coast of Brazil)
• Alizé (northeasterly across central Africa and the Caribbean)
• Alizé Maritime (a wet, fresh northerly wind across west central Africa)
• Amihan (northeasterly wind across the Philippines)
• Bayamo (a violent wind on Cuba's southern coast)
• Bora (northeasterly from eastern Europe to Italy)
• Chinook (warm dry westerly off the Rocky Mountains)
• Etesian (Greek name) or Meltemi (Turkish name) (northerly across Greece and Turkey)
• Föhn (warm dry southerly off the northern side of the Alps and the North Italy)
• Fremantle Doctor (afternoon sea breeze from the Indian Ocean which cools Perth, Western Australia during summer)
• Gilavar (south wind in the Absheron Peninsula)
• Gregale (northeasterly from Greece)
• Habagat (southwesterly wind across the Philippines)
• Harmattan (dry northerly wind across central Africa)
• Halny (in northern Carpathians)
• Khamsin (southeasterly from north Africa to the eastern Mediterranean)
• Khazri (cold north wind in the Absheron Peninsula)
• Kosava (strong and cold southeasterly season wind in Serbia)
• Levanter (easterly through Strait of Gibraltar)
• Libeccio (southwesterly towards Italy)
• Marin (south-easterly from Mediterranean to France)
• Mistral (cold northerly from central France and the Alps to Mediterranean)
• Nor'easter (eastern United States)
• Nor'wester (A wind that brings rain to the West Coast of New Zealand, and warm dry winds (and bad tempers for some) to the East Coast of New Zealand.)
• Santa Ana winds (southern California)
• Simoom (strong, dry, desert wind that blows in the Sahara, Palestine, Jordan, Syria, and the desert of Arabia)
• Sirocco (southerly from north Africa to southern Europe)
• Solano This is another name for the Levanter
• Southerly Buster (rapidly arriving low pressure cell that dramatically cools Sydney, Australia during summer)
• Tramontane (cold northwesterly from the Pyrenees or northeasterly from the Alps to the Mediterranean, similar to Mistral)
• Vendavel (westerly through Strait of Gibraltar)
• Zonda wind (on the eastern slope of the Andes in Argentina)

Meteorological instruments to measure wind speed and/or direction

Wind direction is reported by the direction from which it originates. For example, a northerly wind blows from the north to the south.

Local sensing techniques

• Anemometer (measures wind speed, either directly, e.g. with rotating cups, or indirectly, e.g. via pressure differences or the propagation speed of ultrasound signals)
• Rawinsonde ( GPS-based wind measurement is performed by the probe)
• Weather balloon (passive measurement, balloon position is tracked from the ground visually or via radar; wind profile is computed from drift rate and the theoretical speed of ascent)
• Weather vane (used to indicate wind direction)
• Windsock (primarily used to indicate wind direction, may also be used to estimate wind speed by its angle)
• Pitot tubes

Remote sensing techniques:

• SODAR
• Doppler LIDARs can measure the Doppler shift of light reflected off suspended aerosols or molecules. This measurement can be directly related to wind velocity.
• Radiometry can be used to measure surface roughness from space. This measurement can be used to estimate wind velocity close to the sea surface over oceans.