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Dear Tom,|

Why does it always seem to be windier during the day than at night?

James Skroll,
Des Moines, Iowa

Dear James,

Much of the tendency for it to be windier during daylight hours is driven by sunlight and solar heating. The sun unevenly heats the Earth’s surface which, in turn, imparts uneven warmth to the air immediately above it. For example, a plowed corn field heats far more aggressively than does a highly reflective snow-covered field nearby. The resulting variation in air temperature produces air pressure variations – lower pressure over the warmer corn field and higher pressure above the snow. Wind is nature’s way of attempting to balance these imbalances.

Sun-induced heating disappears with the onset of darkness, and winds fade. But there is another factor at work. Nighttime cooling sets up a temperature inversion, a situation in which cooler, denser air collects at the ground while milder temperatures prevail above. Such inversions effectively shunt large-scale, organized winds away from the surface, forcing organized winds aloft.