Geoscience Reference
In-Depth Information
Box 4.1
The three-cell model of global circulation
The pole to equator temperature gradient, combined with the fact that we live on
a rotating planet, produces several circulation cells in the atmosphere. We can
think of these cells as extending vertically through the lowest 10 to 18 km of the
atmosphere, where temperature gradients are largest. This lowest layer of the
atmosphere is known as the troposphere, and the top of this layer is known as
the tropopause. In the tropics warm, humid air rises in thunderstorms through the
lowest layer of the atmosphere. When this rising air reaches the tropopause, the boundary
between the troposphere and the stratosphere, it stops rising and begins to move laterally
towards both poles. This poleward moving air then begins to sink in the subtropics, near
30ºN and 30ºS. When this sinking air reaches the surface of The Earth some of it is de
ected
towards the equator, completing one cell of the global circulation
-
known as the Hadley cell.
The remainder of this sinking air begins to
flow towards the poles. In the mid-latitudes,
near 50 or 60º latitude, this poleward moving air encounters equatorward moving
air that originated in the polar regions. As these two airstreams collide they are forced to rise.
This rising air ultimately reaches the tropopause and ceases to rise. Some of this air is
de
ected back towards the equator, but sinks near 30º latitude, completing a second
circulation cell known as the Ferrel cell. The remainder of the air that had reached the
tropopause near 50 or 60º latitudes begins moving towards the poles. This air sinks over the
poles, completing the third and
final cell of the global circulation, which is known as the
polar cell.
The equatorward and poleward directed portions of these circulation cells
will transport either cold polar air towards lower latitudes or warm tropical air towards
the poles. Through this circulation the Antarctic
its cold air to the mid-latitudes of
the southern hemisphere, as can be attested to by the residents of places such as Australia,
New Zealand, southern South America and southern Africa, when cold southerly winds
invade their homes and bring the possibility of snow.
'
exports
'
Figure 4.5
Schematic of the north
-
south
and vertical winds that comprise the
global circulation. This schematic is
referred to as three-cell global circulation
model since each hemisphere contains
three circulation cells. (Credit: Elizabeth
Lessard, Cooperative Program for
Operational Meteorology, Education
and Training (COMET), University
Corporation for Atmospheric Research
(UCAR))
60
N
Subpolar lows
30
N
Horse latitudes
0
Doldrums
Horse latitudes
30
N
Subpolar lows
60
N
Sinking air with high pressure
Rising air with low pressure
c
The COMET Program
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