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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. LATITUDINAL CIRCULATION FEATURES The wind belts and the , the Ferrel Cell , and the Polar Cell (the interpretation of the latter two is complex). Note that there is not one discrete Hadley cell, for instance, but several within the Equatorial Zone which shift, merge, and decouple in a complicated process over time. For descriptive purposes, however, they are generally referred to in the singular. Hadley cell See Also: Hadley cell 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 Area s 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. Polar cell See Also: Polar vortex 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 circulates within the Troposphere , limited vertically by the tropopause at about 8 km. Warm air rises at lower latitudes and moves poleward through the upper troposphere at both the north and south poles. When the air reaches the polar areas, it has cooled considerably, and descends as a cold, dry high pressure area, moving away from the pole along the surface but twisting westward as a result of the Coriolis Effect to produce the Polar Easterlies . The outflow from the Polar cell creates Harmonic waves in the atmosphere known as Rossby Wave s. 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 Sink , 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 a winter visit from a . 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; 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 Sink 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. Ferrel cell See Also: Ferrel cell 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 circulations (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 Westerlies can be found beneath the Ferrel cell. Thus, strong high pressure areas which divert the prevailing westerlies, such as a Siberian High (which could be considered an extension of the arctic high), could be said to override the Ferrel cell, making it discontinuous. 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. LONGITUDINAL CIRCULATION FEATURES While the Hadley, Ferrel, and Polar cells are major players in global heat transport, they do not act alone. Disparities in temperature also drive a set of longitudinal circulation cells, and the overall atmospheric motion is known as the zonal overturning circulation. Latitudinal circulation is the consequence of the fact that incident solar radiation per unit area is highest at the heat equator, and decreases as the latitude increases, reaching its minimum at the poles. Longitudinal circulation, on the other hand, comes about because water has a higher specific heat capacity than land and thereby absorbs and releases heat less readily than land. Even at microscales, this effect is noticeable; it is what brings the sea breeze, air cooled by the water, ashore in the day, and carries the land breeze, air cooled by contact with the ground, out to sea during the night. On a larger scale, this effect ceases to be Diurnal (daily), and instead is seasonal or even Decadal in its effects. Warm air rises over the Equatorial , Continent al, and western Pacific Ocean regions, flows eastward or westward, depending on its location, when it reaches the tropopause, and Subside s in the Atlantic and Indian Ocean s, and in the eastern Pacific . |
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