fogged up window
Julie Hambleton
Julie Hambleton
October 15, 2023 ·  4 min read

How to Defog Your Windshield in No Time, Courtesy of a NASA Engineer

One of the most annoying parts about winter is sitting in your car, freezing, waiting for your windshield to defog so you can drive. To top it off, this always seems to take the longest when you’re already late. Learn how to defog your windshield faster with this NASA engineer’s science-based technique.

How to Defog Your Windshield

Fogged part of the car, windshield, part of the front door and mirrors. Morning condensate on a car.
Credit: Shutterstock

Mark Rober is a former NASA engineer who now shares fun science-related videos on his YouTube channel. He reports on everything from how to make an unbreakable safe using an ice cream container, to building the perfect squirrel-proof bird feeder, to experiments to show how germs spread.

One of his videos that is most pertinent to viewers who live in cold climates is about how to defog your windshield faster in the wintertime. Rober has a four-step method that he guarantees will save you time each morning.

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What is Windshield Fog?

Before he gets into how to defog your windshield faster, Rober insists that to understand how his method works you need to know what that window fog is. The fog that completely blanks out your windshield when you get in the car in the winter is actually condensation. Condensation occurs when the air breaches 100% humidity (it’s capacity to store water as vapor) and therefore water leaks out in the form of tiny droplets. (1)

The fog on your windshield is not different from the condensation on the outside of a can of soda, on the bathroom mirror after you shower, or the morning dew on the grass. In order to defog your windshield faster, you need to, in a sense, “wring out” the humid air inside the car and replace it with drier outside air. (1) There are several ways you can achieve this, however, according to Rober’s experiments, this is the fastest one. (1)

Mark Rober’s 4-Step Method for How to Defog Your Windshield

Though some cars have a windshield defog button, most cars have multiple controls that leave you wondering which one control or combination of controls is best to use to defog your windshield.

Rober spent 10 mornings testing out various methods until he found the one that, based on performance, was the fastest window defogger. (1)

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Step 1: Turn Your Heater On Full Blast

heater, air conditioner switching to warm
Credit: Shutterstock

Hot air can hold more moisture than cold air, so by pumping hot air into your car, the air will absorb more of the moisture that is causing the fog, or condensation, on your windshield. (1)

Step 2: Turn Your Air Conditioning On

Finger pressing on Power button on off switch of a Car air conditioning and heating system To turn on the Fan of the AC inside the Car. auto climate control
Credit: Shutterstock

The AC pulls moisture out of the air as the air passes over the cold coil. This helps to effectively remove moisture from the air. (1)

Step 3: Turn Off Inside Circulation

air circulation. hand on air damper positioner to change control air flow switching in car air conditioner control panel.
Credit: Shutterstock

The air outside in the wintertime is cold and doesn’t hold very much moisture. If you bring that dryer, colder air into your car and then heat it up, that air will have a lot more absorption capacity than if you try to use the air already circulating inside your car. (1)

Step 4: Open The Windows Slightly for a Little Bit

Heat, high temperature concept. Closeup of slightly ajar car window for better cooling on hot summer day. Opened vehicle window for air circulation in warm weather
Credit: Shutterstock

If it is raining or extremely cold you might not be able to use this step, but if you can, cracking your windows brings in more of that cold outside air into the car. Even just a few seconds of this will help to exchange the humid air inside your car with the dry air from the outside. (1)

Rober explains that your results will be somewhat varied depending on the weather outside and what kind of car you drive, but in general, he says this is the best method for most people. (1)

Windshield Fog Prevention

At the end of his video, Rober also gives two options to help prevent the windshield from getting as fogged up in the first place. The first is filling up a sock with kitty litter and placing it on the dashboard of your car. Kitty litter is highly absorptive and will absorb some of the moisture out of the air. (1)

The second is using an anti-fog window treatment or shaving cream. He did another experiment comparing an untreated portion of the window to a portion treated with anti-fog and another with shaving cream. (1)

The results? The anti-fog treatment worked incredibly well, however, the shaving cream worked just as well. His advice? Save yourself a bit of money and just go with the shaving cream. (1)

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  1. Defog your windows TWICE as fast using SCIENCE- 4 easy steps.Youtube. Mark Rober. January 5, 2016.