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A similar air mass rising on the other side of the equator forces those rising air masses to move poleward. The rising air creates a low pressure zone near the equator. As the air moves poleward, it cools, becomes denser, and descends at about the 30th parallelcreating a high-pressure area. The descended air then travels toward the equator along the surface, replacing the air that rose from the equatorial zone, closing the loop of the Hadley cell.
The poleward movement of the air in the upper part of the troposphere deviates toward the east, caused by the coriolis acceleration a manifestation of conservation of angular momentum.
Global circulation patterns - Met Office
At the ground level, however, the movement of the air toward the equator in the lower troposphere deviates toward the west, producing a wind from the east. The winds that flow to the west from the east, easterly wind at the ground level in the Hadley cell are called the Trade Winds. Though the Hadley cell is described as located at the equator, in the northern hemisphere it shifts to higher latitudes in June and July and toward lower latitudes in December and January, which is the result of the Sun's heating of the surface.
The zone where the greatest heating takes place is called the " thermal equator ". As the southern hemisphere summer is December to March, the movement of the thermal equator to higher southern latitudes takes place then. The Hadley system provides an example of a thermally direct circulation. The power of the Hadley system, considered as a heat engine, is estimated at tera watts.
There it subsides and strengthens the high pressure ridges beneath. A large part of the energy that drives the Ferrel cell is provided by the polar and Hadley cells circulating on either side and that drag the Ferrel cell with it.
It might be thought of as an eddy created by the Hadley and polar cells.
In the upper atmosphere of the Ferrel cell, the air moving toward the equator deviates toward the west. Both of those deviations, as in the case of the Hadley and polar cells, are driven by conservation of angular momentum. As a result, just as the easterly Trade Winds are found below the Hadley cell, the Westerlies are found beneath the Ferrel cell. The Ferrel cell is weak, because It has neither a strong source of heat nor a strong sink, so the airflow and temperatures within it are variable.
For this reason, the mid-latitudes are sometimes known as the "zone of mixing. The weaker Westerlies of the Ferrel cell, however, can be disrupted. The local passage of a cold front may change that in a matter of minutes, and frequently does.
As a result, at the surface, winds can vary abruptly in direction. But the winds above the surface, where they are less disrupted by terrain, are essentially westerly. A strong high, moving polewards may bring westerly winds for days.
The Ferrel system acts as a heat pump with a coefficient of performance of Polar vortex and Polar easterlies The Polar cell is a simple system with strong convection drivers. Though cool and dry relative to equatorial air, the air masses at the 60th parallel are still sufficiently warm and moist to undergo convection and drive a thermal loop. As it does so, the upper level air mass deviates toward the east. When the air reaches the polar areas, it has cooled and is considerably denser than the underlying air.
It descends, creating a cold, dry high-pressure area. At the polar surface level, the mass of air is driven toward the 60th parallel, replacing the air that rose there, and the polar circulation cell is complete.
As the air at the surface moves toward the equator, it deviates toward the west. Again, the deviations of the air masses are the result of the Coriolis effect.
The air flows at the surface are called the polar easterlies. The outflow of air mass from the cell creates harmonic waves in the atmosphere known as Rossby waves.
Global circulation patterns
These ultra-long waves determine the path of the polar jet streamwhich travels within the transitional zone between the tropopause and the Ferrel cell. By acting as a heat sink, the polar cell moves the abundant heat from the equator toward the polar regions. 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 drive the weather in their domain.
The sheer volume of energy that the Hadley cell transports, and the depth of the heat sink that is the polar cell, ensures that the effects of transient weather phenomena do not only have negligible effect on the system as a whole, but — except under unusual circumstances — do not form.
There are some notable exceptions to this rule.
In Europe, unstable weather extends to at least the 70th parallel north. The polar cell, terrain and Katabatic winds in Antarctica, can create very cold conditions at the surface, for instance the lowest temperature recorded on Earth: While the Hadley, Ferrel, and polar cells whose axes are oriented along parallels or latitudes are the major features of global heat transport, they do not act alone.
Atmospheric circulation - Wikipedia
There is a big temperature difference between the poles and equator, but our global circulation provides a natural air conditioning system to stop the equator becoming hotter and hotter, and poles becoming colder and colder.
The global circulation Over the major parts of the Earth's surface there are large-scale wind circulations present. The global circulation can be described as the world-wide system of winds by which the necessary transport of heat from tropical to polar latitudes is accomplished.
In each hemisphere there are three cells Hadley cell, Ferrel cell and Polar cell in which air circulates through the entire depth of the troposphere. It is the part of the atmosphere where most of the weather takes place.
Hadley cell The largest cells extend from the equator to between 30 and 40 degrees north and south, and are named Hadley cells, after English meteorologist George Hadley. From the tops of these storms, the air flows towards higher latitudes, where it sinks to produce high pressure regions over the subtropical oceans and the world's hot deserts, such as the Sahara desert in North Africa.
Ferrel cell In the middle cells, which are known as the Ferrel cells, air converges at low altitudes to ascend along the boundaries between cool polar air and the warm subtropical air that generally occurs between 60 and 70 degrees north and south.
This often occurs around the latitude of the UK, which gives us our unsettled weather. The circulation within the Ferrel cell is complicated by a return flow of air at high altitudes towards the tropics, where it joins sinking air from the Hadley cell.
The Ferrel cell moves in the opposite direction to the two other cells Hadley cell and Polar cell and acts rather like a gear. However, the spin of the Earth induces an apparent motion to the right in the northern hemisphere and left in the southern hemisphere.
This deflection is caused by the Coriolis effect caused by the spin of the Earth and leads to the prevailing westerly and southwesterly winds often experienced over the UK. Polar cell The smallest and weakest cells are the Polar cells, which extend from between 60 and 70 degrees north and south, to the poles. Air in these cells sinks over the highest latitudes and flows out towards the lower latitudes at the surface.
Without the Coriolis effect, winds would run north to south or south to north in each of the cells, but the Coriolis effect 'drags' the winds, giving an east or west component to the wind direction, depending on the hemisphere you are in.
Warm, moist air from the tropics gets fed north by the surface winds of the Ferrel cell.