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How to stop car windows from fogging
Updated 20 Apr 2021
The dreaded fog up - one of the most dangerous situations to be in whilst driving. Typically, window fogging will happen in the worst weather conditions, which means you’re not only dealing with poor weather but you can’t see where the road is either.
We all know why the windows fog up when the car is stationary, but why do they fog up when you’re driving and what can you do to avoid it and eliminate it as quickly as possible if it does happen?
The science behind window fog
Window fog occurs when a layer of mist forms on the inside of a car's windows. It happens when a car’s windows are colder than the dewpoint temperature. Dewpoint is defined by the temperature to which the atmosphere needs to be cooled to reach saturation, with saturation being 100% relative humidity. When the air drops below the dewpoint, condensation forms.
To avoid condensation, you need to remove moisture from inside the car. Sounds simple enough, right?
WINDSCREEN CRACKED? Here’s what to do
Preventing window fog
Clean: Water particles find it easier to stick to dust and contaminants on the window. If the windows are clean, it will reduce fog build up.
Leaks: Check your car for water leaks. The more moisture sitting inside the car, the higher chance of fogging.
Anti-fog product: There are some products on the market that are designed to reduce fog by applying a coating to the window.
Remove excess water: Don’t bring water into the car. If it is raining, and it’s preventable, try not to bring wet clothes and umbrellas into the car. Perhaps throw them in the boot if you can.
Dehumidifier: This might sound silly but they do actually make car dehumidifiers. These can be placed under the windscreen to remove moisture. You could even go as far as keeping some baking soda or kitty litter in the car!
Vent hair: Recirculate the air. Vents should be open and it's a good idea to also crack a window before parking to allow the car to release any humid air.
How to remove window fog
Air conditioning (A/C): It’s not just for hot days. If you have it, use it. The idea is to remove moisture, so ramp up the A/C.
Heater: If you don’t want to freeze with the A/C on, then put the heater on and direct heat to the windows, this will warm the windows above the dewpoint.
Demister cloth: Keep a cloth in the car to wipe the moisture away from the windscreen. Some people suggest a whiteboard eraser is a great tool to remove fog. Whatever works for you. Make sure you are not making things worse by using a material that is contaminating the windscreen. And remember that safety comes first, you should always pull off the road before attempting to wipe the windscreen clear.
Recirculate off: Make sure the recirculate vents are open to allow fresh air in and humid air out.
I hope that helps with the fogging-up predicament. Basically, dry air is your friend and moisture is not when it comes to seeing clearly on a day the dewpoint drops. Drive safe and if at any point you can’t see clearly, it’s time to pull over and wait until you can.
Is your car’s A/C and heater working? If not, have it checked by a professional AutoGuru mechanic to get it operating efficiently to help prevent the fog!
Rachel spent her early adult life around cars, motorsport and hands-on with her own cars. This interest moved into various careers within the Automotive industry. Joined with her passion for writing, Rachel loves putting the two together to share her experience, so we can all become AutoGuru’s.