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To understand the earth's surface wind systems, we must study the global system of barometric pressure distribution. Once we grasp the patterns of isobars and pressure gradients, the prevailing or average winds can be predicted.
World isobaric maps are constructed to show average pressures and winds for the two months of seasonal temperature extremes over large landmasses - January and July. Because observing stations lie at various altitudes above sea level, their barometric readings must be reduced to sea level equivalents, using the standard rate of pressure change with altitude. When this has been done, and the daily readings are averaged over long periods of time, small but distinct pressure differences remain from place to place.
2. Main article
If 1013 mb is taken as standard sea-level pressure, readings higher than this will frequently be observed in middle latitudes, occasionally up to 1040 mb or higher. These pressures are designated as high. Pressures ranging down to 982 mb or below are low.
3. References
In the equatorial zone is a belt of somewhat lowers than normal pressure, between 1011 and 1008 mb., which is known as an equational trough. Lower pressure is made conspicuous by contrast with belts of higher pressure lying to the north and south and centered on about 30° N and S°. These are the subtropical belts of high pressure. In the southern hemisphere this belt is clearly defined but contains centres of high pressure, termed pressure cells. In the northern hemisphere in summer the high-pressure belt is dominated by two oceanic cells one over the eastern Pacific, the other over the eastern North Atlantic. Average pressures exceed 1026 mb in the centre of the cells.
Poleward of the subtropical high-pressure belts are broad belts of low pressure, extending roughly from the middle latitude zones to the arctic zone but centered and intensified in the Sub arctic zone at about the 60th parallels of latitude. In the Southern hemisphere, over the continuous expanse of Southern ocean the sub arctic low pressure belt is especially defined with average pressure as low as 984 mb. The polar zones have permanent centre of high pressure known as the polar high. The pressure belts shift seasonally through several degrees of latitude just as do the isotherm belts that accompany them.
