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Whereas the troposphere is from 5 to 12 mi (8 to 20 km) thick. What is the nature of airflow at these higher levels? Slowly moving high-and low-pressure systems are found aloft, but that these are generally simple in pattern with smoothly curved isobars. Winds, which may be extremely strong and follow the isobars closely, move counterclockwise around the lows (northern hemisphere), but clockwise around the highs.

Two systems dominate. One is the system of westerlies blowing in a complete circuit about the earth from about latitude 25° almost to the poles. At high latitudes these westerlies constitute a circumpolar whirl, coinciding with a great polar low-pressure centre. Toward low latitudes the pressure rises steadily at a given altitude, to form two high-pressure ridges at latitudes 15° to 20° N and S. These are the high-altitude parts of the subtropical highs, but are shifted somewhat equatorward. In the high-pressure zones, wind velocities are low, just as in the horse latitudes at sea level. Between the high-pressure ridges is a trough of weak low pressure, in which the winds are easterly, comprising the second major circulation system of the globe, termed the equatorial easterlies. At lower elevation their influence spreads into somewhat higher latitudes as the trade winds.

Circulation in equatorial and tropical latitude zones resolves itself into two circuits, one in each hemisphere. Air rises over the equatorial zone but subsides in the subtropical cells, forming the Hadley cell. Some of the subsiding air escapes poleward, into the westerlies. Thus the circulation of low latitudes is a heat engine.

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