Ask Ellen: Why is it colder on clear nights?

Ask Ellen

We tend to notice in the winter something that happens year round, the massive drop in temperatures on calm, clear nights.

Our coldest lows usually happen on the quietest nights

While it can seem counter-intuitive, it often isn’t the snowiest or windiest weather that brings the coldest lows, but the nights following. Typically after a snow storm moves through, an area of high pressure scoots in on its heels. This allows skies to clear and winds to settle.

Temperatures drop on cold, clear and calm nights the most because it is an ideal setup for the ground to release long-wave radiative energy back into space. Each day, the sun heats the ground, and that radiative energy then slowly releases back into the air. This is why the coldest temperatures are usually close to dawn. It takes time for the sun to heat up the ground and allow that heat in the soil to re-emit back into the air.

Clouds act like a blanket, keeping us warm at night

At night, the coldest temperatures are reached when the long-wave radiative emission is maximized. This happens when the winds are still and the clouds are minimal. Clouds act like a giant blanket, trapping some of that long-wave radiative energy closer to the earth. When clouds are gone, warmth is allowed to escape back into space more easily.

Clouds during the day have an opposite effect, shielding the earth from some of the incoming warmth of the sun.

Wind will often mix the air so much that it prevents rapid cooling, which is why the coldest conditions happen on the calmest, starriest nights.

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