What do high pressure and low pressure mean, and what is the difference?

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Dear Tom,
What do high pressure and low pressure mean, and what is the difference?

Mike McGarry,
Fayetteville, Arkansas

Dear Mike,
“High” and “low” pressure refer to air pressure, the pressure exerted on all things by the weight of the air. Air pressure values typically range between 28.75 inches of mercury and 30.50 inches of mercury (the height of a mercury column supported by air pressure at sea level), but can be higher and lower in extreme situations. High pressure means the air pressure at a location is higher than at all surrounding locations; low pressure, the air pressure is lower. The distance between high and low pressure centers on weather maps is typically several hundred miles, but this can vary greatly depending on the weather situation. High pressure areas are usually associated with fair, dry weather; lows with clouds and precipitation.

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