Wind, in the most general sense, is the movement of air. 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, lasting eons. 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). 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 an 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.
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 this will be strongly modified by the Coriolis effect.
While all winds are the movement of air more or less parallel to the Earth’s surface, they come in a variety of forms. 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 winds that result from pressure differences in surface airmasses at the middle latitudes, and there are winds that come about as a consequence of geographic features such as oceans, lakes, mountains, and deserts. Mesoscale winds are those which act on a local scale, such as gust fronts. At the smallest scale are the winds which blow on a scale of only tens to hundreds of metres and are essentially unpredictable, such as dust devils and microbursts. Finally, there are special-case winds that come about as a consequence of local geography.
Wind can also shape landforms, via a variety of eolian processes.
Global 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.
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 Old English ‘trade’, 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 equatorial 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.
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 always flow clockwise around a high pressure area and counterclockwise around a low pressure area (the reverse 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 out runs parallel to the isobars. Wind following this path is known as geostrophic wind. It is rare, however, to find things quite so tidy. 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 nature, isobars are almost always curved. The result is that a wind moving parallel to the isobars encounters a third force, the centripetal force. This is the force which tends to keep a body in motion moving in the same direction. The effect of this force, though not a force in itself, is called the centrifugal force, and acts to counteract the Coriolis force (coincidentally also the effect of a force rather than a force in itself) and decrease the wind speed. This much more common situation results in what is known as a gradient wind.
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.
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 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 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.
Special winds are winds which blow under only certain circumstances. These may result from differential heating, from barriers to airflow, or from gravitational effects.
Differential heating is the motive force behind land breezes and sea breezes (or, in the case of larger bodies, lake breezes), also known as on- or off-shore winds. Water is a rapid absorber/radiator of heat, whereas land not only absorbs heat more slowly but 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 released more quickly by the water. Air contacting water cools. Over the land, heat is still being released into the air, which rises. This convective motion draws the cool sea air in to replace the rising air, resulting in a sea breeze. During the day, the roles are reversed. The land, cooled from a night of radiation, continues to soak up heat long after the heat capacity of the water has been reached. Warm air over the water rises, pulling cool air from inland to replace it. And so it goes.
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 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 foehn, California’s infamous Santa Ana wind, and the Spanish 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 foehn, which undergo lifting by mountain ranges before descending and warming on the lee side.