Defog Your Car Windows the Fast and Easy Way … With Science!

4 Settings to Defrost Your Windshield Faster

Defog Your Car Windows the Fast and Easy Way … With Science!


When you find your pre-owned BMW’s windshield covered in a layer of fog or frost you might be wondering how to defrost car windows quickly so you can be on your way. Using the window defroster for your car is key, but there are other steps you can take when wondering how to defrost car windows fast in Bedford.

Should you open your windows, or keep them sealed? Is hot air, or cold air better? Former NASA engineer Mark Rober tested all the variables over ten days of controlled trials to come up with the best combination of defroster settings–find out the science-based tips for how to defrost car windows with Tulley BMW of Manchester.

How to Defrost Car Windows

  • First, turn the front defogger on high. If your vehicle has a rear windshield defroster, turn that on as well.
  • Turn the fan on high. This maximizes the amount of drying air.
  • Turn the temperature on high. Warmer air holds more moisture and can dry the fog faster.
  • Turn the air conditioning on. This makes the fan act as a dehumidifier.
  • Turn off recirculated air. Outside air is dryer than your humid car on a cold day.

These settings should cut down the time you have to wait until your windows are clear.

Remember not to drive when your windshield is frosted over, or you might just end up needing our collision repair center.

Additional Window Defrosting Solutions

If you’re wondering how to defrost car windows quickly, and the steps listed above aren’t working as quickly as you’d , we have a few additional tips on how to defrost car windows. Try making this solution to clear the windshield in less than a minute:

  • Mix 1/3 part water and 2/3 part isopropyl or rubbing alcohol.
  • In the morning, spray this solution on your windshield and the ice should disappear instantly.
  • If you’re looking for other ways to defrost your windshield, try other pre-made solutions at your local dealership or look into a portable defroster.

You can also prevent frost from getting on your front windshield in the first place with accessories the FrostGuard windshield cover. The product hooks over your vehicle’s side-view mirrors and the frost-blocking fabric covers your windshield. If frost gets on the shield, you simply unhook it, shake it onto the ground, and stow in your car or garage.

Learn More at Tulley BMW of Manchester

If you’re ready to learn more about how to defrost car windows fast or what other solutions are available to use, contact our service center. Our technicians can help fix the heat in your vehicle if that’s what’s causing the issue as well as help you pick the right window defroster to melt ice quickly. Contact Tulley BMW of Manchester today!

More from Tulley BMW of Manchester

  • Anyone with even a cursory knowledge of motorsports recognizes and respects the M. BMW’s M models have dominated GT and road racing circuits since the 1970s, and the M badge still represents exclusivity and supreme performance across the full line of BMW model series. BMW now offers an M model in 7 of its series…Read More
  • Salt is necessary for life, although it is possible to get too much salt, as anyone with high blood pressure knows. The substance may be necessary for life, but it is not good for a person’s car. A person has to protect their car from the corrosion salt can cause. Cars, people, need to…Read More
  • Parents use safety seats for their children to keep them safe when they are in a motor vehicle. Although car seats are manufactured to offer superior protection in the event of a collision, if they are not installed correctly, then they will not be effective. To learn how to install a child safety seat correctly,…Read More


Find Your Vehicle ×

Type New Used Certified Pre-Owned Year Make Model


Saved Vehicles ×

You don't have any saved vehicles!

Look for this link on your favorites:


Once you've saved some vehicles, you can view them here at any time.


Defogging Your Car Windshield with Psychrometrics

Defog Your Car Windows the Fast and Easy Way … With Science!

A lot of people don't understand the best way to defog their car windshield in wet, cool weather.  About 30 years ago, I was on a group trip on a hired shuttle bus.  The outdoor temperature was probably in the 40s Fahrenheit and it was raining.  Perfect conditions for fogging up the interior glass, right? 

Unfortunately, the shuttle driver didn't understand psychrometrics, the study of the mixture of dry air and water vapor.  She chose setting for the bus's HVAC system that not only didn't work well, but made us all uncomfortable for the whole five-hour trip.  She did have one setting right, though.

In the decades since then, I've figured out the optimal way to defog a car windshield on those days and I'll share it with you here.

  I've written about all the principles behind the optimal method in this blog before so let's put them together in a lesson on applied psychrometrics.

  And at the end, I've got a video that shows you exactly what's going on and why the settings work.

1.  Cold surfaces condense water vapor

The air in the car is fairly isolated from the outdoors.  If people are inside breathing, the humidity can rise and the dew point can easily go above the temperature of the front and back windshields and the side windows, too.  That's when those glass surfaces fog up.

As you know if you've been reading this blog for a while, that gives you two pathways to fix the condensation problem:

  1. Warm up the windows.
  2. Reduce the humidity, and dew point, inside the car.

2.  Cold air is dry air

The amount of water in the vapor phase depends on temperature and availability of water.  As the temperature goes higher, more water from liquid and adsorbed phases goes into the vapor phase.  Conversely, as the temperature drops, water vapor condenses the vapor phase and the dew point goes down.

And that's why it's better to talk about the actual quantity of water vapor with dew point (or humidity ratio for the psychrometric geeks out there).  Cold air can have very high relative humidity.

  It can be 100%.  But I've shown before that when you bring cold, 100% relative humidity air inside and warm it up, the relative humidity of that air drops tremendously.

  And that's why we say cold air is dry air.

The best way to defog your car windshield

Now we know enough to choose the proper settings.  Ready?

  1.  Set the fan speed as high as it goes.
  2.  Set the car's HVAC system to the highest temperature setting.
  3.  Set the air direction to blow on the windshield.
  4.  Turn the air conditioner on.  (Yes, really!  Remember, you've got the temperature set to high.)
  5.  Set the fresh air/recirculate option to fresh air.
  6.  Open the windows a bit if it's not raining or too cold.

Those settings will optimize your windshield defogging.  You obviously want to move a lot of air and you want high temperature air so you can heat the windshield surface above the dew point and evaporate the condensed moisture that's already there.  These settings keep the moisture inside the car but put it back into the vapor phase instead of fogging up your windows.

The reason you want the air conditioner on is that it removes moisture from inside the car.  Once you get that water into the vapor phase, the AC can condense it and remove it from the inside of the car.

You want the fresh air setting because cold air is dry air.  That helps you send humid air to the outside and bring in colder air that has a lower relative humidity when you heat it up.  And that's the same reason you want to open the windows a bit if you can.

Now, here's that excellent video I promised.  It's by a guy named Mark Rober, a mechanical engineer who worked at NASA for nine years.  But he's not your typical engineer using techno-jargon.  He explains all this really well.

So there you have it.  If you've got fogged-up windows in your car, now you know what to do…un my shuttle drive 30 years ago.

  I don't know all the settings she used and maybe there was a lockout on one part of it, but she made us all freeze on that long drive from Tallahassee to Clearwater that chilly day.

  She did turn the AC on but she made the mistake of thinking that a low temperature setting would work better than a high temperature setting.  She may well have used the recirculate mode, too, because I think she still had a window fogging problem most of the way back to Clearwater.

Thanks to Anthony Cox for sending me the link to the Mark Rober video.

Related Articles

Make Dew Point Your Friend for Humidity

Two Rules for Preventing Humidity Damage

Cold Air Is Dry Air

NOTE: Comments are moderated. Your comment will not appear below until approved.