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What causes the fog on car windshields on a cold day

It is an all too common occurrence for people living in cold climates. You wake up in the morning, get into your car parked in the chilly weather overnight and before you can turn on the engine, the windshields start fogging up forcing you to turn on the defrosters / air-conditioning and just sit and wait as the fog clears. But what causes the fogging? How come it usually happens only after you enter the vehicle?

Humidity in cold vs hot air:
The key to understand fogging in a car is the fact that hot air can hold more water (humidity) than cold air. In fact, hot air can hold up to twice the amount of moisture than cold air. You might have experienced this in the form of dry skin or chapped lips in the cold weather which is a direct result of the cold air being less humid and so readily taking away your body moisture.

Another key phenomena is that when hot humid air comes in contact with a colder surface, the water Vapor Condenses Forming a fog. This is also easily observable when we exhale on a cold day. The air coming out of our body is warm and humid and as soon as it comes in contact with the cold air outside, the vapor condenses forming a visible fog.

What happens when I enter my car:
When the car is parked for an extended period in cold weather, the humidity levels on the outside and inside reach an equilibrium. However, as soon as the driver enters and starts breathing inside the car, the air inside the car gets warmer and humid because of that person's warm breath and any other moisture they might have carried in (like water droplets on a jacket, vapor from a hot coffee etc.). This relatively hot and humid air starts condensing and fogging up as soon as it comes in contact with the cold windshields. 


While it is unavoidable, a few tips to deal with foggy windshields - 
  • If you can avoid it, don't carry any hot beverages into the car as you enter.
  • Carry an absorbent piece of cloth to wipe away the fog on the windshield. Not only is this more time efficient than running defrosters but is also environmentally friendlier.
  • Run the air-conditioning on cold setting first and gradually rise the temperature. As discussed above, cooling down the air in the car reduces the chances of your humidity being carried by air and condensing on the wind shield. By the time you turn it on to warm setting, the air conditioning system would have already cleared the humidity.

This post first appeared on ScienceLet, please read the originial post: here

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What causes the fog on car windshields on a cold day


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