Is air “cleaner” during cold weather?

Dear Tom,

I can see great distances on cold nights; farther than on warm nights. Is air “cleaner” during cold weather?

James Millory,
New Buffalo, Michigan

Dear James,

Cold air often appears cleaner, and may, in fact, be cleaner, but not always. In the Arctic, problems with visibility and air cleanliness can develop on cold nights because of sharp temperature inversions (warm air over cold air), which trap pollution in a shallow layer near the surface. Residents of Fairbanks, Alaska, for example, occasionally deal with episodes of noxious ice fogs.

The transparency of the atmosphere is also highly dependent on its moisture content. Cold air is drier and clearer appearing. The moister air of the warm season can hold visibility-impeding water droplets and ice crystals, but much colder air doesn’t support them in the same numbers.

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