Due to the spherical shape of the earth, areas close to the equator heat up faster than the air at the poles, as the same amount of radiation energy spreads out over a much larger area than close to the equator. As nature strives to reach a state of equilibrium, large-scale motion in the atmosphere (and in the oceans) starts taking place to balance the energy deficit at the poles and the excess at the equator. This is the basis of the global atmospheric circulation.
Due to the rotation of the earth, air motions (winds) are deflected on their way from low to high latitudes. The resulting pseudo force, the so-called Coriolis force, causes the winds in the northern hemisphere to be deflected to the right, and winds in the southern hemisphere to be deflected to the left. Poleward flowing air masses therefore are deflected to the east, causing the characteristic westerly winds prevailing in the mid-latitudes.
The dominant wind systems close to the equator are the trade winds blowing from the east. They are connected to ascending warm and moist air close to the equator, where the solar radiation is strongest. The trade winds at the surface converge together in the Inter Tropical Convergence Zone (ITCZ). Due to the ascending air masses and the connected drop in pressure close to the ground, an area with low pressure, known as the Equatorial Trough, is formed. At around 30° north and south, the air is descending. This decent warms and dries the air, thus forming the large desserts around the globe. The whole circulation with ascent over the ITCZ and descent in the so-called horse latitudes is called the Hadley circulation.
Close to the pols, descending air masses prevail, which leads to higher pressure close to the ground, resulting in the polar high pressure system. Here the Coriolis force causes the polar winds to blow from the east. These winds are relatively dry, as cool air can absorb less water vapor than warm air.
W. Roedel (2011): Physik unserer Umwelt: Die Atmosphäre. Springer-Verlag.
H. Kraus (2004): Die Atmosphäre der Erde: Eine Einführung in die Meteorologie. Springer-Verlag.
Text: Dr. Susanna Mohr, Karlsruhe Institute of Technology