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The large-scale structure of the atmospheric circulation varies from year to year but the basic structure remains fairly constant. However, individual weather systems - midlatitude depressions, or tropical convective cells - occur "randomly", and it is accepted that weather cannot be predicted beyond a fairly short limit: perhaps a month in theory, or (currently) about ten days in practice (see Chaos theory). Nonetheless the average of these systems - the climate - is quite stable.
The wind belts and the jet streams girdling the planet are steered by three convection cells: the Hadley cell, the Ferrel cell, and the Polar cell. Note that there is not one discrete Hadley cell, for instance, but several within the equatorial zone which shift, merge, and decouple in a random process over time. For descriptive purposes, however, they are generally referred to in the singular.
The Hadley cell mechanism is well understood. The atmospheric circulation pattern that George Hadley described to provide an explanation for the trade winds matches observations very well. It is a closed circulation loop, which begins at the equator with warm, moist air lifted aloft in equatorial low pressure areas to the tropopause and carried poleward. At about 30°N/S latitude, it descends in a cooler high pressure area. Some of the descending air travels equatorially along the surface, closing the loop of the Hadley cell and creating the Trade Winds.
Though the Hadley cell is described as lying on the equator, it should be noted that it is more accurate to describe it as following the sun’s zenith point , or what is termed the " thermal equator," which undergoes a semiannual north-south migration.
The Polar cell is likewise a simple system. Though cool and dry relative to equatorial air, air masses at the 60th parallel are still sufficiently warm and moist to undergo convection and drive a thermal loop . Air masses circulate meridionally within the troposphere, limited vertically by the tropopause at about 8 km, with (on average) polewards motion aloft and equatorial motion closer to the surface. When the air reaches the polar areas, it has cooled considerably, and descends as a cold, dry high pressure area, twisting eastward as a result of the Coriolis effect to produce the Polar Easterlies . The outflow from the Polar cell creates harmonicIn acoustics and telecommunication, the harmonic of a wave is a component frequency of the signal that is an integral multiple of the fundamental frequency. For a sine wave, it is an integral multiple of the frequency of the wave. For example, if the freq waves in the atmosphere known as Rossby wavePhysical oceanography Rossby (or planetary waves are large-scale motions in the ocean or atmosphere whose restoring force is the variation in Coriolis effect with latitude. The waves were first identified in the atmosphere in the 1939 by Carl-Gustaf Arvids. These ultra-long waves play an important role in determining the path of the jet stream, which travels within the transitional zone between the tropopause and the Ferrel cell. By acting as a heat sinkA heat sink is a device, usually made of metal, brought into contact with the hot surface of a component, such as a microprocessor chip, in order to aid the cooling of that component through thermal dissipation by conduction and convection. Heat sinks are, the Polar cell also balances the Hadley cell in the Earth’s energy equation.
It can be argued that the Polar cell is the primary weathermaker for regions above the middle northern latitudes. While Canadians and Europeans may have to deal with occasional heavy summer storms, there is nothing like the arrival of a winter visit from a SiberiaSiberia ( Russian: , common English transliterations: Sibir Sibir' is a vast region of Russia and northern Kazakhstan, constituting all of northern Asia, and extending eastward from the Ural Mountains to the Pacific Ocean, and southward from the Arctic Ocn high to give one a true appreciation of real cold. In fact, it is the polar high which is responsible for generating the coldest temperature recorded on Earth, -89.2°C at Vostok II Station in 1983 in, where else, Antarctica.
The Hadley cell and the Polar cell are similar in that they are thermally direct; in other words, they exist as a direct consequence of surface temperatures. As well, their thermal characteristics override the effects of weather in their domain. The sheer volume of energy the Hadley cell transports, and the depth of the heat sinkA heat sink is a device, usually made of metal, brought into contact with the hot surface of a component, such as a microprocessor chip, in order to aid the cooling of that component through thermal dissipation by conduction and convection. Heat sinks are that is the Polar cell ensures, that the effects of transient weather phenomena are not only not felt by the system as a whole, but— except under unusual circumstances— are not even permitted to form. The endless chain of passing highs and lows which is part of everyday life for mid-latitude dwellers is unknown above the 60th and below the 30th parallels.
These atmospheric features are also stable, so even though they may strengthen or weaken regionally or over time, they do not vanish entirely.
The Ferrel cell, theorized by William Ferrel (1817-1891), is a secondary circulation feature, dependent for its existence upon the Hadley cell and the Polar cell. It behaves much as an atmospheric ball bearing between the Hadley cell and the Polar cell, and comes about as a result of the eddy circulation s (the high and low pressure areas) of the midlatitudes. For this reason it is sometimes known as the "zone of mixing." At its southern extent, it overrides the Hadley cell, and at its northern extent, it overrides the Polar cell. Just as the Trade Winds can be found below the Hadley cell, the WesterliesThe westerlies are the prevailing winds in the middle latitudes between 30 and 60 degrees latitude, blowing from the high pressure area in the horse latitudes towards the poles. The winds are predominantly from the southwest in the northern hemisphere and can be found beneath the Ferrel cell.
While the Hadley and Polar cells are truly closed loops, the Ferrel cell is not, and the telling point is in the Westerlies, which are more formally known as "the Prevailing Westerlies." While the Trade Winds and the Polar Easterlies have nothing over which to prevail, their parent circulation cells having taken care of any competition they might have to face, the Westerlies are at the mercy of passing weather systems. While upper-level winds are essentially westerly, surface winds can vary sharply and abruptly in direction. A low passing to the north or a high passing to the south (from a Northern Hemisphere frame of reference) maintains or even accelerates a westerly flow; the local passage of a cold front may change that in a matter of minutes, and frequently does. A strong high passing to the north may bring easterly winds for days.
The base of the Ferrel cell is characterized by the movement of air masses, and the location of these air masses is influenced in part by the location of the jet stream, which acts as a collector for the air carried aloft by surface lows ( a look at a weather map will show that surface lows follow the jet stream). The overall movement of surface air is from the 30th latitude to the 60th. However, the upper flow of the Ferrel cell is not well defined. This is in part because it is intermediary between the Hadley and Polar cells, with neither a strong heat source nor a strong cold source to drive convection, and in part because of the effects on the upper atmosphere of surface eddies, which act as destabilizing influences.
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