This morning it was extra cold (15 degrees F!). It would have been cold anyway because an arctic air mass arrived over the weekend, but it was extra cold because the sky was mostly clear last night. If we'd had lots of cloud cover we'd have been a little warmer.
The reason for this is not what you'd expect. Traditionally we've heard that cloud cover acts like a blanket to hold the heat in. The illustration above plays to that notion by showing heat arrows bouncing off the clouds. But it ain't exactly so. Believe it or not this illustration is wrong.
The truth is that we're warmer under cloud cover because the clouds radiate their own heat which warms the air below them. You've seen this principle in action if you've parked your car under a leafy tree on a frosty night and found your windshield frost-free the next morning though the open ground has frost. The tree radiated heat to keep your car just a little warmer than the open air.
The clouds do not “hold the heat in”. They absorb the heat, and radiate their own heat in all directions. ...If you're camping, and you sleep under a tree, you will escape most of the dew compared to your buddies, who slept right out under the stars. The tree did not catch the dew, it just radiated energy to the ground around you, and kept it warmer. Warmer ground, less dew!
Click on the image to read more of Dan's Scientific Facts That Aren't True.
Clouds may blanket us but they aren't blankets.
(traditional image of heat bouncing off clouds from Dan's Wild Wild Science Journal: Scientific Facts That Aren't True by Dan Satterfield)