Clouds aren't really a 'blanket' on warm nights

You sometimes hear that "clouds act like a blanket" to keep temperatures from falling as low as they otherwise would if the sky were clear. It's true that the effect of clouds can be like a blanket. But they work in a different way. A blanket keeps us warm by keeping the warm air our body creates from rising, which would allow cool air to reach our skin. The action of clouds is more complicated. Everything radiates infrared energy, or heat. A thing's temperature determines how much energy and what kinds. The Earth is always radiating away infrared energy. Various molecules of gas in the air, especially water vapour and carbon dioxide, absorb some of this infrared energy. They, in turn, radiate energy away in all directions, including back to Earth. If the Earth had no atmosphere, its average temperature would be close to -18 degrees Celsius instead of the 15 degrees it actually is. The tiny water droplets that make up clouds radiate even more heat than the gases in clear air. For one thing, the tiny droplets absorb some frequencies of infrared energy that the air's gases don't. As the energy is absorbed, it heats the droplets, which causes them to radiate energy. Then too, the cloud has more matter - the cloud droplets - to radiate heat in all directions, including back toward Earth, than clear air.

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