The Fastest Way to Defog Your Car Windshield, According to Science

January 6, 2016 | Joanne Kennell

rainin on a car windshield
Photo credit: kaboompics/Pixabay (CC0)

These steps can reduce the amount of time you wait by half.

Waiting for your windshield to defrost or defog is one of the more frustrating things the winter season has to offer — especially if you are already running late.  But now, thanks to Mark Rober, there is a way to make the process twice as fast.

Before we get into the process, we should probably understand the science behind why fog forms in the first place.  Fog is basically tiny droplets of water.  Fog forms on surfaces when warm, moist air comes into contact with cooler objects and condenses on them — and this can happen on both the inside and outside of a car windshield.  The source of this moisture is water vapor or humidity in the air.

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Fog condenses on the inside of the window because the air outside is cooler, and colder air lowers the relative humidity (the amount of water vapor in the air).  It is the same process as seeing your breath on a cold day.  The source of this moisture inside the car is usually from our breath, damp floor mats, and even hot take-out food!

And when these surfaces continue to cool — through the loss of infrared radiation — to a temperature that is colder than the dewpoint of the air and also below freezing, we get frost..

In the video below posted below, Rober explains why and how he conducted his experiments.

So what does Rober suggest to reduce the amount of time you wait for your windshield to defrost?

1. Put the heat on full blast.  This is because heat holds more water, meaning the air requires more water vapor before it will condense.

2. Turn the A/C on.  Turning on the A/C removes any excess moisture in the air.

3. Make sure the inside air circulation is off.  We want to incorporate dry outside winter air, rather than recirculating the humid air already in the car.

4. Crack open the windows for just a little bit.  This will exchange the humid air in the car with the drier air outside.

Hopefully you will find Rober’s tips and tricks helpful and useful.


Editor's Note (January 15): The content of the article has been updated to specify that these measures are effective against fog, not necessarily frost. The title and main image have also been updated to reflect these changes.

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