Science Questions

Why is it that if water freezes at zero,we get frost at three degrees?

Why is it that if water freezes at zero, we get frost at three degrees? Alan in Lowestoft

What I think is going on is that the ground is looses heat by radation. So it can see the clear cold sky above it on a clear night and the ground radiates away lots of its energy very quickly and goes down to zero or below zero very effectively, because the ground is good at giving up energy. But the air and the atmosphere sitting above the ground is effectively transparent to the radiation  so it doesn't absorb or emit radiation well, so it doesn't cool down nearly as fast or insulate the ground very much.  So the air loses it's energy much more slowly than the ground does. This means that the air can be a three degrees but the ground is now lower that, say zero or even minus three degrees. So when the warm damp air meets the air, it cools down and the moisture condenses out as frost on the ground even though the air temperature is still not quite freezing.

February 2007