Science Questions

Why does the windscreen mist up faster when it rains?

I was sitting in the car with the engine off and no heater or cooling fan on and the weather's dry or overcast but not raining. It takes ages when breathing normally inside the car for the windows to mist up. When it rains the windscreen mists up faster even though there's no obvious change in temperature. Why should that be? Edward James, Milton Keynes

There's two things to think about here.  Number one, when it's raining the air's pretty moist.  Lots of water in the air even if you're sitting inside the car the airs going to have a lot of water vapour in it.  It's going to condense onto the inside of your windscreen.  The second point to think about is that when it's raining outside you've got cold water hitting the outside of your windscreen.  Previously you'd just have some air going over the windscreen.  The cold water's just going to take the heat away much better than the air does.  That's really encouraging any water on the inside of the car to condense on the inside of the screen.  It'll mist up your windscreen very quickly.

March 2008