Listener Question: Cloud Blankets |
 Dave Thurlow, Host |
Hi, I'm Dave Thurlow and this is The Weather Notebook. The insulating ability of clouds. That's the subject of our latest listener question:
"This is Valerie Sullivan. I listen to The Weather Notebook on 88.7, WBFO in Buffalo, NY. And my question is, last month on our forecast, the forecaster said that because there was no cloud cover, it was going to be colder than usual. And I would think that cloud cover would make things colder, with the full sun shining that it would be warmer, so I was wondering if you could clarify that. Thank you."
Well thank you Valerie. You know clear skies don't often mean warm temperatures for a couple of reasons. The first has to do with something called radiational cooling. At night when there is no sunlight, when there's no warmth coming in, there is warmth going out. The earth radiates heat to space, that's why nights tend to be chilly. But if there are clouds in the sky they act precisely like a blanket. The heat tries to radiate to space but it can't because of the cloud blanket and the earth below stays relatively toasty.
Another reason that clouds and milder temperatures go hand in hand is that to get clouds in the first place, you need rather mild air, for that time of year. So, air that's above you when there are clouds tends to be a little bit warmer so it can hold the moisture that ultimately produces the clouds. So therefore Valerie, when you hear about a forecast for cloudy skies it's often just a little bit warmer, especially in the winter.
So Radiational cooling can't happen when it's cloudy and cloudy air is generally warmer to begin with. And that's the answer to your question. Our show is underwritten by Subaru and the National Science Foundation.