Explaining why clear, calm nights produce the coldest mornings

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Monday morning hit a frigid low of 17 degrees below zero. It was even colder than when we had arctic air in place just a week ago. So why did we fall so far this time?

The overall air mass in place plays a big role in our temperatures, especially during the day.

But when night time rolls around, it’s a different story, especially this time of the year.

We cool off at night because we lose incoming solar radiation during the day, and the ground then starts to radiate energy it absorbed back out.

But that outgoing radiation doesn’t just bolt off into space, some of it gets absorbed by air just off the surface, warming it up just a bit relative to the air near the surface.

And when the skies are clear and the winds are calm, the temperatures just near the surface drop off essentially uninhibited.

But when there’s a stiff breeze, like last week, that all changes.

The winds stirring around do just that, stir up the low levels of the atmosphere.

The warmer air that’s just off the surface mixes with the colder air at the surface, meaning the near-surface atmosphere in general just is a little more temperate.

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