Why does my car windscreen mist up only when it rains?

11 January 2009

Question

Why does my car windscreen mist up only when it rains?

Answer

Dave - What's happening when your car windscreen mists up is you've got lots of moisture in the air inside your car. If the car windscreen is cold enough then it will be below the dew point and below the temperature at which water can condense on it. You will get condensing on the windscreen until eventually you get little droplets making it hard to see through. There's various things which will affect this. One of them is the amount of moisture in the air. If you've got rain it's very humid and you're probably wet. You're probably heating up water and it's evaporating off you so it makes the air very moist. The other big one is how cold the glass is. If it's raining then rain's probably hitting the glass, taking a lot of the energy out of it and cooling it down. Probably two reasons: one is that there's more moisture in the air and the other is that it's cooled down.

To try and stop this from happening, you should try this Kitchen Science Experiment! - http://www.thenakedscientists.com/HTML/kitchen-science/exp/stopping-fogg...

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