Dear Tom,
Why does the wind die away at sunset? I have always wondered about that.
James Tiller, Moline, Ill.

Dear James,

It doesn’t happen during stormy weather or when other factors  keep nighttime winds blowing, but as fair days draw to a close, brisk daytime winds die. This happens because the ground begins to cool when it is no longer receiving heat from sunlight, and the air immediately above it cools also. As the shallow layer of cool, heavy, ground-hugging air grows deeper, surface winds diminish or cease. Wind continues to blow a few hundred feet or more above the ground, and it continues do so all through the night. When the next morning’s sun heats the ground again, the wind aloft mixes down to the surface and ground-level winds start to blow once again.