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The Spherical Shape of the Earth: Climatic Zones
The Sun heats the ground or ocean surface most intensely in the tropical zone. The heated air
rises, and as it rises, it cools, and as it cools it dumps its moisture as rain. This belt
of converging air masses, called the doldrums due to low air and water circulation sometimes
causing ships to struggle to escape the region, includes some of the rainiest areas on Earth.
The cooled, now drier air is forced by continuously rising air to move out of the way, and
so it moves towards the temperate latitudes.
Such air from the tropics meets air moving down from the poles at about 30° N and S, called
the horse latitudes, where it settles. Here the sinking air compresses, warms, and absorbs
moisture from the surface. This is why the desert belts lie in the horse latitudes. This
warm, dry air is displaced by more sinking air and so some of it returns back to the equatorial
zone, and some returns to the poles. Such cycling air between low and mid-latitudes defines
a Hadley Cell.
A similar cell forms between the horse latitudes and the stormy polar fronts at 60° N and 60° S,
where warm temperate air moving polewards meets very cold air rolling down from the pole. The
lighter warm air is forced to rise over the denser cold air, which chills it and forces precipitation.
From this polar front, air returns both equatorwards and polewards. Air immediately over the pole
sinks. While it is not warm, it is extremely dry (only centimeters of snow every year). From
the poles, air within the polar cap streams back towards the polar front.
Thus, six belt-like Hadley Cells circulate air from pole to pole and establish patterns of
climate over the planet. The cells are also characterized by specific patterns of wind flow,
a function of the Coriolis force generated by the spin of the Earth. In the temperate zone
between the horse latitudes and the polar front, the prevailing westerlies dominate air circulation.
In the tropics, the easterly trade winds dominate. Winds around the poles are also easterly.
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