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HADLEYCELL The Hadley cell, named after
George Hadley, is a global
scale tropical atmospheric
circulation that features air
rising near the Equator,
flowing poleward at a height
of 10 to 15 km above the
earth's surface, descending
in the subtropics, and then
returning equatorward near
creates the trade
deserts and the
Hadley cell are
have air sinking
at roughly zero
to 30 degree
DRIVINGFORCE The major driving force of
atmospheric circulation is the
uneven distribution of solar
heating across the Earth, which is
greatest near the equator and
least at the poles.
The atmospheric circulation
transports energy polewards,
thus reducing the resulting
The mechanisms by which this is
accomplished differ in tropical and
Hadley cells exist on either side of the equator. Each cell encircles the globe
latitudinally and acts to transport energy from the equator to about the 30th
latitude. The circulation expose the following:
• Warm, moist air converging near the equator causes heavy precipitation. This releases latent
heat, driving strong rising motions.
• This air rises to the tropopause, 10–15km above sea level, where the air is no longer buoyant.
• Unable to continue rising, this sub-stratospheric air is instead forced poleward by the
continual rise of air below.
• As air moves poleward, it both cools and gains a strong eastward component due to the
Coriolis effect and the conservation of angular momentum. The resulting winds form the
subtropical jet streams.
• At this latitude, the cool, dry, high altitude air begins to sink. As it sinks, it warms adiabatically,
decreasing its relative humidity.
• Near the surface, a frictional return flow completes the loop, absorbing moisture along the
way. The Coriolis effect gives this flow a westward component, creating the trade winds.
Part of the air rising at 60° latitude diverges at high altitude toward the poles
and creates the polar cell.
The rest moves toward the equator where it collides at 30° latitude with the
high-level air of the Hadley cell.
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.
The Ferrel cell, theorized by William Ferrel is a secondary circulation feature,
whose existence depends upon the Hadley and polar cells on either side of it.
It might be thought of as an eddy created by the Hadley and polar cells.
Ferrel’s model was the first to account for the westerly winds between
latitudes 35° and 60° in both hemispheres.
The Ferrel cell, however, is still not a good representation of reality because it
requires that the upper-level mid-latitude winds flow westward; actually the
eastward-flowing surface winds become stronger with height and reach their
maximum velocities around the 10-km level in the jet streams.
WESTERLIESWINDS The air of the Ferrel cell that descends
at 30° latitude returns poleward at
the ground level, and as it does so it
deviates toward the east.
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
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.
The Ferrel system acts as a heat pump.
This cell shares its southern, descending side with the Hadley cell to its south.
Its northern rising limb is shared with the Polar cell located between 50
degrees N to 60 degrees N and the North Pole, where cold air descends.
There are three mirror image circulation cells in the Southern Hemisphere.
The largest cells extend from the equator to between 30 and 40 degrees
north and south, and are named Hadley cells, after English meteorologist
Within the Hadley cells, the trade winds blow towards the equator, then
ascend near the equator as a broken line of thunderstorms, which forms the
Inter-Tropical-Convergence Zone (ITCZ).
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.
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
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 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.
In this cell the surface wind would flow from a southerly direction in the
northern hemisphere. 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 and leads to the
prevailing westerly and south-westerly winds often experienced over the UK.
The smallest and
weakest cells are the
Polar cells, which
extend from between
60 and 70 degrees
north and south, to
Air in these cells sinks
over the highest
latitudes and flows
out towards the
lower latitudes at the