Wind is caused by differences in the atmospheric pressure. When a difference in atmospheric pressure exists, air moves from the higher to the lower pressure area, resulting in winds of various speeds. The closer the high and low pressure areas are together, the stronger the "pressure gradient", and the stronger the winds. On weather maps, lines of constant pressure are drawn which are called "isobars". These isobars are usually labeled with their pressure value in millibars (mb). The closer these lines are together, the stronger the wind. On a rotating planet, air will also be deflected by the Coriolis effect, except exactly on the equator.
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