The weight of air pushing against the earth is what weathermen call air pressure. This weight changes from place to place and from time to time in the same place. These changes are sometimes caused by changes in temperature.
When the sun’s heat warms the air, it makes the air currents rise skyward. Since warm air weighs less than cold air, the warm air pushes less against the earth, or exerts low pressure. When the heavier, cold air pushes with more weight against the earth, it exerts heavier, or high pressure.
High pressure, or highs, warm the air and evaporate the clouds, bringing clear, dry, and sunny weather. Low pressure, or lows, provide cool air for clouds to form and bring stormy weather.
In the Northern Hemisphere, the winds around a high-pressure area always travel clockwise. The winds around a low-pressure area travel counterclockwise.
Knowing this, weathermen, or meteorologists, can predict wind direction and speeds at which pressure areas travel. This helps them figure out when a storm will hit certain parts of the country.
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