Why are there less cumulus clouds in winter?

Dear Tom,

I enjoy seeing cumulus clouds that populate our skies, but I don’t see them as much during the winter season. Why?

— William Cho, Chicago


Dear William,

Cumulus clouds frequently occur in Chicago’s sky during the warmer portion of the year but do not appear as often during the winter.

That’s because they consist of updrafts of warm air that have risen to the atmosphere’s “condensation level” and beyond. The sun heats the ground and the ground heats the air above.

After enough daytime heating, warm surface air (from the ground up at least several hundred feet) rises in drafts or columns that build into cumulus clouds.

In the winter, however, the sun is much lower in the sky and the heat it provides (per area) is much less. Surface air is often cooler than overriding air, so it does not rise.