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Mans environment at the earth's surface depends for its quality as much upon atmospheric motions as it does upon the flow of heat by radiation and conduction. Air in motion represents a form of kinetic energy. Winds also transfer energy to the surface of the sea, as wind-driven waves.
But air in motion has another, more basic role to play in the planetary environment. Large-scale air circulation transports heat, both as sensible heat and as latent heat present in water vapour.
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Because of the global energy imbalance-a surplus in low latitudes and a deficit in high latitudes-atmospheric circulation must transport heat across the parallels of latitude from the region of deficit to the regions of surplus. Figure shows this transport in schematic form. Notice that circulation of ocean waters is also involved in transport of sensible heat. But this oceanic circulation is a secondary mechanism, driven largely by surface winds.
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Conversely, large-scale sinking motions in the atmosphere lead to aridity and the occurrence of deserts. In this way the earth's surface becomes differentiated into regions of ample fresh water and region of water scarcity.
Winds and the pressure gradient force - Winds are dominantly horizontal air motions with respect to the earth's surface. (Dominantly vertical air motions are referred to by other terms, such as updrafts or downdrafts.) To explain winds, we must first consider barometric pressure and its variations from place to place.
Thus, the surfaces of equal barometric pressure, or isobaric surfaces, are horizontal; in a cross section of a portion of the atmosphere at rest, these surfaces appear as horizontal lines.
The isobar is a line showing the location on a map of all points having the same barometric pressure.
The change in barometric pressure across a horizontal surface constitutes a pressure gradient. Where a pressure gradient exists, air molecules tend to drift in the same direction as that gradient. This tendency for mass movement of the air is referred to as the pressure gradient force. The magnitude of the force is directly proportional to the steepness of the gradient. Wind is thus the horizontal motion of air in response to the pressure gradient force.
