Dear Tom,
It is usually windy during the day, but calm at night. Why?

Jimmy Marcine, Beloit, Wis.

Dear Jimmy,
Wind is nature’s way of trying to equalize air pressure differences in the atmosphere. The sun unevenly heats the Earth’s surface and this imparts uneven warmth to the air immediately above it, causing air pressure differences. Air pressure will be lower in the warmer air than in the cooler air.

For example, in the winter the air over a snow-covered surface will be cooler than air over an adjacent lake: Air over Chicago after a snow storm compared to air over Lake Michigan. The resulting temperature difference produces lower air pressure in the warmer air over Lake Michigan and higher air pressure in the colder air over the city. Wind, blowing from higher to lower air pressure, is nature’s way of trying to balance these imbalances.

However, wind dies away toward sunset because sun-induced heating disappears with the onset of darkness. A “temperature inversion” develops in which cooler and denser, heavier air collects at the ground while warmer, uncooled air prevails above. Such temperature inversions effectively shunt large-scale organized winds away from the surface, leaving them still blowing aloft (a few hundred feet or more above the ground).