The other night, around 1 am, I noticed frost on my car. The temperature at the time was well above freezing—like, 37° or so. I feel like this happens all the time, even though, according to physics, it should be impossible. In the immortal words of Sarah Palin, WTF? —Slush Puppie
I sympathize with your confusion, Slush. This phenomenon so flies in the face of our native intuitions about weather that plenty of folks will tell you it simply can’t happen—you must have read the thermometer wrong.
The native intuitions usually include some vague idea that the ground gets frosty cold because cold air blows in from a cold place (the North Pole, maybe?) and makes it cold. If that air is cold enough, it will chill the ground below the freezing point of water, making frost possible.
By this theory, the ambient temperature would need to be well below freezing for frost to form—how else could the ground get cold enough? What our intuition completely misses is that, ultimately, it’s not the air that chills the ground. Really, it’s the ground (and the trees, and the hills; Earth in general) that gets cold and chills the air. And it does this by—brace yourselves—radiating heat into outer space.
Yes, THAT outer space. As sweaty, warm-blooded creatures, we’re mostly aware of losing heat to the air; we don’t think much about emitting it as infrared radiation. In the universe at large, however, radiative cooling is a much bigger deal. Think about it: There’s no air on the moon, but it still manages to get plenty cold there at night. Our planet’s ability to radiate its excess heat into the cosmos is the only reason it hasn’t been fried to a cinder (yet).
Normally, we don’t really notice this effect. We’ve got the sun warming things back up every day, frequent clouds to reflect heat back to Earth even at night, and breezes to stir the air and keep everything at pretty much the same temperature: all close enough to what our intuition predicts.
But sometimes, on a calm, cloudless night, you can see the real process at work—like that time the thermal mass of your car was able to cool radiatively far faster than the air around it, becoming cold enough to freeze water even though the thermometer said it was impossible. In the immortal words of Rosanna Arquette: That was trippy!
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