You Don’t Fog up Your Car Windows Properly
If your windshield is wet, you are using the wipers. If it’s dirty, you press a button that sprays it with this blue liquid. And if it’s ice cold, you heat it up to defrost it. But what if you have a foggy windshield and fog inside ? Someone says to use heat, someone says to use alternating current, but what is the right way?
This is usually confused, so if you’re not sure, you’re not alone. This weekend, a tweet about the dilemma went halfway viral, and many replies only made things more confusing.
Turn on the heating and air conditioning
I’ll get to a more detailed scientific explanation in a minute, but the short answer is: to quickly get rid of windshield fogging in cold weather, you need to turn on the heating and air conditioning.
Yes, you can do both at the same time. If you have a dial that regulates temperature, set it to the hottest setting. Then look for the button that turns the air conditioner on or off. Click it to enable it.
Some cars have a button that does everything for you. My car has a defrost button on the back that turns on the wires on the rear windshield and a defrost button on the front that does the following:
- Directs the blower to the windshield
- Turns the fan on at full speed and raises the temperature
- Includes AC
- Turns off recirculation
This is exactly what you need to do if you manage the controls yourself: turn on the heating, turn on the air conditioner and make sure that the air is blowing at full speed on the windshield (look for the windshield icon, not the icon with arrows pointing at a person). Turning off recirculation helps because you will be drawing in dry air from outside instead of recirculating slightly moist air inside.
Why it works
There are two things happening here: in the short term (seconds to minutes) and in the long term (the rest of the journey).
Heat prevents fogging
In the long run, you can prevent fogging by making sure both sides of the windshield are at the same temperature. Mist builds up on your windshield for the same reasons that a cold drink “sweats” 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 (since glass is colder than warm air).
This means that in winter you will get condensation from warm indoor air onto your still cold windshield. If you can warm up the windshield enough, it will stop pulling moisture out of the air.
In summer it’s the other way around. If you’ve cooled the inside of your car and it’s 100 degrees outside, you can get moisture from the outside air condensing on your windshield. However, this is not a defogger situation: the condensate is outside, so you simply wipe it off with wipers.
AC dries up the mist that ‘s already there
So why isn’t heat alone enough to do the job? Well, first of all, sometimes it’s too cold to keep the windshield warm enough to prevent fogging. But more importantly, you need to get rid of windshield fogging, not just prevent future fogging. That’s why you turn on the air conditioner.
Air conditioners don’t just cool the air; they dry it out. When you turn on the heating and air conditioning at the same time, you get hot and dry air. This starts to heat up the windshield and also dry out the moisture. Turn the A/C and heater on full blast and often the entire windshield is cleared in seconds.
How to minimize fogging of windows
Because windshield fogging is caused by moisture inside the car, it will help keep the interior of the car dry. If a lot of snow and slush gets into the car, clean it.
You can also start warming up the windshield before driving. If you have a remote starter, great! But even if not, the first step in getting your car ready for winter should be to start the engine and warm it up. Warm up the windshield while cleaning and clean the outside.
Finally, consider cleaning the inside of your windshield because clean glass doesn’t fog up as easily. No, not with your hands or those fast food napkins you found under the seat; oils for the skin and various fats and dirt will only exacerbate the problem. Use a suitable glass cleaner such as Windex.