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Why do windshields fog up?

One warm humid day, I pulled out of my garage and the windshield fogged up.  I fiddled with the controls but to no avail. Then I realized the fog was on the outside. I got rid of it with my wipers. I could use some mental defogging on this issue. Like so many things, it's about heat and wetness.


Temperature is short hand for how much molecules are moving. Warm air seems to be able to “hold” more moisture than cold. But this is not a chemical reaction — the molecules in the air are not hanging on to individual water molecules. It’s mostly a state of matter thing.

When it’s warmer, a higher proportion of water can become a gas instead of a liquid. And warmer air has more space between the molecules so more water molecules can fit in there. A clothes dryer cranks up the heat so more water evaporates.


When it's humid, more water is available to turn into water vapour. The air you breathe out is humid. It just came out from inside (if not topologically) your body. Usually windshield fogging more an issue with a bunch of people in the vehicle, or a few people are breathing heavily...


At cooler temperatures, a higher proportion of water turns into a liquid than a gas and you get condensation, clouds in cold air (check out this cloud in a room) or fog on glasses if you step from the cold into a warm humid room, or on your windshield on a cold day. In my case, the windshield was cool from sitting in the garage overnight and then met with warm, humid air.

Surface Tension

Water forms tiny droplets because the water molecules prefer other water molecules to a foreign body like glass. If you rub soap on your hockey visor or spit into your scuba mask, you change the surface properties so water “likes” it better.


According to my car manual (which, don't worry, I read it at home, not while driving), the defog setting automatically chooses outside air instead of recirculating. Recirculating air will increase the humidity and make it more difficult to defog.

For a fast defog, use the maximum heat with the maximum air. This gets annoying after a while. If you turn it off, then it may fog up again. To keep it away, switch to a less annoying amount of heat and air. This source suggested that if the blowing air is cooler than the windshield, then the air will warm on contact and more moisture will evaporate. But that might take you out of your comfort zone. In any case, get your navigator to fiddle with this, so you can pay attention to the road.

Learn more 

Try making a Cloud in a Bottle on Science World Resources.