60 Kinematics and Dynamics of a Point Mass
60°/• / Polar Easterlies /
/
//y'Temperate Westerlies/ /
30°/ Calms of Cancer
// Easterly Trade Winds //
Doldrums
X^Easterly Trade Winds \^\ ,
Calms of Capricorn
\\ \Temperate Westerlies\\/
\
Polar Easterlies 'Fig. 2.1.8 Atmospheric WindsThe general wind pattern on the Earth is as follows (see Figure 2.1.8).
Warm air rises vertically from the surface and is deflected by the Corio-
lis force, resulting in easterly trade winds, temperate westerlies, and polar
easterlies. The deflection is to the right in the Northern Hemisphere and to
the left in the Southern Hemisphere, so that the patterns in the two hemi-
spheres are mirror images of each other. Three regions of relative calm
form: the doldrums around the equator, calms of Cancer around the 30°
parallel in the Northern Hemisphere, and calms of Capricorn around the
30° parallel in the Southern Hemisphere.
Let us discuss the formation of the easterly trade winds in the Northern
Hemisphere. The Sun heats the surface of the Earth near the equator.
The air near the equator also gets warm, becomes lighter, and moves up,
creating the area of low pressure near the equator and causing the cooler
air from the north to flow south. The flow of the cooler air from the North
creates the area of low pressure at high altitudes, deflecting the rising warm
air from the equator to the north. The Coriolis force deflects this flow to the
East. At higher altitudes, the air cools down. Cooler, denser air descends
around the 30° parallel and flows South back to the lower pressure area
around the equator. The Coriolis force deflects this southward flow to the
West. In the stationary regime, this circulation produces a steady wind
from the North-East, the easterly trade winds.