When you drive in the winter, you have to deal with a lot of foggy windshields. What's the best way to make that fog go away? Some people will tell you to blast the heat, others to turn on the air conditioning. Who's right? And when should you just hit that "defrost" button instead? I'll explain.

To defog in cold weather, 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. My car has a windshield 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.

How to defog your windshield in hot weather

If you're reading this on a sweltering summer day, just turn on your windshield wipers. The fog is on the outside of your windshield, so you can just wipe it away.

What is the difference between defrosting and defogging?

OK, some definitions. Fog is made of tiny droplets of water that have condensed onto the windshield. You'll find this anytime you have a difference in temperature between two sides of the glass. The fog forms on the warmer side (which is the inside of your car, in winter).

Frost, on the other hand, is a layer of ice. You'll see it most commonly on the outside of your car, and it's what you scrape off with an ice scraper. In very cold weather, the fog on the inside of your windshield can freeze, giving you frost on the inside. Treat this the same as fog.

The rear defrost button in your car heats up a set of wires embedded in the glass. They melt the ice on the back window.

The front defrost button in your car will defrost the windshield by heating it up, and defog it by drying out the air to evaporate the fog. It's both a defogger and a defroster.

Check your car's manual for specifics on what settings your car has and how they work. The rear defrost button is very common, even in old cars; the front defrost button is a newer development but still commonly available.

The science of defogging

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 (and frost)

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 winter, 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.

A/C 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 A/C 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.

If you've heard the advice to open your windows, that can help for a similar reason: a blast of cold dry air will help to dry up the fog. But the air conditioning will usually do the same job quicker, without making your entire car cold. So I'd stick with the A/C.

How to prevent foggy car windows

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 winter 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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