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© NASA
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High and low pressure areas on a weather map of North America.
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Why is the weather in high-pressure areas usually fair? Why is the weather
in low-pressures areas usually cloudy and stormy? Most weather maps show
areas, labeled with an H, where the atmospheric pressure is relatively
high, and areas labeled with an L where the atmospheric pressure is relatively
low. The isobars around such areas are closed curves with the approximate
shape of circles. High-pressure areas are places where the atmosphere
is relatively thick. Winds blow outward from these areas, although in
a spiraling way. As air leaves the high-pressure area, the remaining air
sinks slowly downward to take its place. That makes clouds and precipitation
scarce, because clouds depend on rising air for condensation. High-pressure
areas usually are areas of fair, settled weather. Low-pressure areas are
places where the atmosphere is relatively thin. Winds blow inward toward
these areas. This causes air to rise, producing clouds and condensation.
Low-pressure areas tend to be well-organized storms.
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