Why Do Windshield Fog Up?

windshield tintingWhat can you do when your windshields fog up after starting to drive? Why does this happen and what can you do about it? There are a variety of reasons why your windshield would fog including humidity, heat, and condensation.


Humidity causes a buildup in water that turns into water vapor. You breathe the humid air. This air comes out from inside your body or off of it. Windshield fogging tends to be more of a problem when there is more than one person in the car since there is more breathing.


Temperature is actually molecules moving. More moisture is held with warm air than cold. However, this isn’t a chemical reaction. The molecules aren’t holding on to water molecules. More water becomes gas when it’s warmer than it does a liquid. And, there’s more space between each molecule with warm air therefore, the water molecules can fit into this space.


Moist, warm air condensing when it hits your car’s colder windshield causes condensation. This usually happens in the early morning. This is also why on humid, warm days when you have your car air conditioner running you’ll see condensation form at the windshield’s base where it meets the cool air.

When there’s 100 percent relative humidity, if the temperature decreases more, the water needs to find its way out someplace. On a morning that’s cool outside when there’s fog or dew on the ground, that’s because the moist, warm air contacts the cold ground.

How to Clear Away the Fog on your Windshield

You can clear fog in many ways. And although there’s no special way of doing it that works best each time, the two that are more likely to work better are as follows.

  • Put your defroster on a high setting.
  • Set the temperature to hot.
  • Turn off the recirculation setting.
  • Turn on the air conditioner.
  • Crack your windows.

Dry the Air Out

Your best option is to run your air conditioner, depending on your comfort level. This method works well when the outside temperatures are above freezing and damp, wet humans getting into the car causes the fog. While cold is best when running your A/C, you can still run the air at a warmer temperature and get rid of the fog.

Evaporate the Water Vapor

For an easy and fast fix, turn the heat on and run it through your defrosters. This causes the condensation to evaporate kind of like the moisture on your clothes evaporating when you turn the clothes dryer on. This method works better when there are colder temperatures and you are catching the fog buildup early.

When your windshield is extremely foggy, you can use a small squeegee or dry cloth to clear up the fog quickly and then turn on the cold A/C. Slowly increase the air’s temperature if you find the A/C is too cold for you until you dry out the air inside your car.

Keep in mind that when you’re drying air that’s inside your vehicle, you need to pull in the outdoors air rather than simply circulating inside air. These days, most defrosters already do this.

Now, that you know what to do when your car windshield fogs, do you know what to do if your windshield has a chip or a crack? Call 7 Star Auto Glass today!