Science Questions

I've been hearing a lot about when you get frost on your car first thing in the morning and people saying that it's the dawn dip. Before it gets light there's no frost and after the sun comes up there's frost on the windscreen. What exactly do they mean by the dawn dip? Tony in Norwich

Well I have to say that it's a term I've not actually heard. But the coldest part of the night is just before the sun comes through. We've lost the daytime moisture and there's still a bit of heat in the ground, but it's just as we go through to the very early hours of the morning that we see the coldest temperatures, so about 5 or 6 o'clock. You can also get a lot of condensation and dew at that time of the morning and that's what can cause lots of these problems as well. So it's a combination of the moisture and the cold temperatures that would have caused the windscreen to freeze out.

September 2006