If your windshield is wet, you use the wipers. If it’s dirty, you hit the button that squirts it with that blue fluid. And if it’s icy, you blast heat at it to defrost it. But what if your windshield is foggy, and the fog is on the inside? Some people say to use heat, some to use the AC — but which is correct?
It’s common to be confused about this, so if you’re unsure, you’re not alone. A tweet on the dilemma went semi-viral this weekend, with many of the replies only making the situation sound more confusing.
Came to the comments to get an answer and im still just as confused as I was before. Glad we’re in this together.
— Brittany Valenzuela (@BRITTANY_COHEN3) January 20, 2023
Turn on the heat and the air conditioning
I’ll go into a more detailed, scientific explanation in a minute, but the short answer is that to defog the windshield quickly in cold weather, you want to turn on the heat and the air conditioning.
Yes, you can do both at once. If you have a dial that adjusts temperature, put it on the hottest setting. Then look around for a button that turns the AC on or off. Click it so it’s on.
Some cars have a button that does everything for you. In my car, there is a rear defrost button that turns on the wires in the back windshield, and a front defrost button that does the following:
- Points the blower at the windshield
- Turns the blower on full blast and raises the temperature
- Turns the AC on
- Turns recirculation off
This is exactly what you want to do if you’re managing the controls yourself: heat on, AC on, and make sure the air is getting blown full blast on the windshield (look for the windshield icon instead of the one with the arrows pointing at the person). Turning off recirculation helps because you’ll be pulling in dry air from outdoors instead of recirculating the slightly humid air inside.
Why this works
There are two things going on here, in the short term (seconds to minutes) and the long term (the rest of your drive).
Heat prevents fogging
In the long term, you can prevent fogging by making sure both sides of the windshield are the same temperature. The fog on your windshield accumulates for the same reasons as a cold drink “sweating” on a hot day. When one side of the glass is significantly colder than the other, moisture from the warm air will condense on the warmer side of the glass (because the glass is cold compared to the warm air).
This means that in the cooler months, you’ll get condensation from the warm indoor air onto the still-cold windshield. If you can warm up the windshield enough, it will stop being able to pull moisture out of the air.
In summer, it’s the opposite. If you’ve chilled the inside of the car but outdoors it’s 100 degrees, you can get moisture from the outside air condensing on your windshield. This is not a defogger situation, though: The condensation is outside, so you just wipe it off with the windshield wipers.
AC dries up the fog that is already there
So why isn’t heat alone enough to do the job? Well, for one thing, sometimes it’s just too cold to keep the windshield warm enough to prevent fogging. But more importantly, you need to defog the windshield, not just prevent future fog. That’s why you turn on the air conditioning.
Air conditioners don’t just chill the air; they dry it out. When you run the heat and the AC at the same time, you get hot, dry air. This starts warming the windshield while also drying up the moisture. Put the AC and heat on full blast, and often the whole windshield will clear up in seconds.
How to get foggy windows less often
Since windshield fog is caused by moisture inside the car, keeping the inside of your car dry will help. If you’ve tracked a lot of snow and slush into the car, clean it out.
You can also start warming up the windshield before it’s time to drive. If you have a remote starter, great! But even if not, the first step in getting your car ready to drive in the cooler months should be turning on the car and getting it warmed up. Put the heat on the windshield while you brush and scrape the outside.
And finally, consider cleaning the inside of your windshield, because clean glass doesn’t fog as easily. No, not with your hands or with those fast-food napkins you found under the seat; skin oils and miscellaneous grease and dirt will only make the problem worse. Use a proper glass cleaner like Windex.
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