Dear Tom,
In meteorology, what is high pressure?
—Mel Thompson, Watseka, Ill.
Dear Mel,
“Pressure” refers to air pressure at ground level, unless otherwise specified. “High pressure” and “low pressure” are relative terms, with the former always denoting a greater air pressure than the latter. On weather maps, a “high” (high pressure system) represents an area, usually at least a few hundred miles across, in which air pressure readings are everywhere greater than at all surrounding locations.
Average air pressure at sea level is 29.92 inches of mercury. That is, the air pressure at the bottom of a column of mercury 29.92 inches in height exactly equals the pressure at the bottom of a column of air extending from sea level to the top of the atmosphere.