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Dear Tom,

Why does the barometric pressure usually fall when it snows?

Sylvia Mallen,
Omaha, Nebraska

Dear Sylvia,

The function of a barometer is to measure atmospheric pressure, the force exerted on a given unit of area due to gravity’s pull on the air directly above. Barometric pressure usually falls when snow (or any organized precipitation) approaches, for several reasons. First, temperatures often increase when precipitation moves in. Increased air temperature decreases the air’s density, thereby decreasing the force exerted by that air, and the barometric pressure falls. Second, warmer snow-bearing air is also relatively moist, and moist air is less dense than dry air. Third, pockets of strong winds in the jet stream (known as jet streaks) draw air aloft, contributing to air pressure reduction at the surface.