Geoscience Reference
In-Depth Information
January
July
North Pole
North Pole
20
20
15
15
10
10
5
5
S.L.
S.L.
60°N
60°N
30°N
30°N
Equator
Equator
0
30°S
30°S
60°S
60°S
1010
1010
800
800
600
400
600
400
200
0
200
0
Wind speed (m s
-1
)
Easterly flow
South Pole
South Pole
Figure 7.17
Mean zonal wind speeds (m s
-1
) calculated for each latitude and for elevations up to more than 20km.
Note the weak mean easterly flow at all levels in low latitudes dominated by the Hadley cells, and the strong upper
westerly flow in mid-latitudes, localized into the subtropical jet streams.
Source: After Mintz; from Henderson-Sellers and Robinson (1986).
to as
thermally direct
cells. Warm air near the
equator was thought to rise and generate a low-
level flow towards the equator, the earth's rotation
deflecting these currents, which thus form the
northeast and southeast trades. This explanation
was put forward by G. Hadley in 1735, although
in 1856 W. Ferrel pointed out that the conserva-
tion of angular momentum would be a more
effective factor in causing easterlies, because the
Coriolis force is small in low latitudes. Poleward
counter-currents aloft would complete the low-
latitude cell, according to the above scheme, with
the air sinking at about 30
latitude as it is cooled
by radiation. However, this scheme is not entirely
correct. The atmosphere does not have a simple
heat source at the equator, the trades are not
°
