Why Do Windshields Fog?

You’ve likely been there. You jump into your vehicle and after you’ve been driving for a few minutes, your windows beginning fogging up. Why does this happen and what can you do about it?

There are a variety of reasons why your windshield would fog including:


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

This is a form of condensation, as tiny water droplets are accumulating on the inside of the windows. In this case, you may notice that the fog wipes away from inside the window when you touch it. Although they are not the only place where condensation may be occurring, windows are the most obvious place where condensation can become visible.


Temperature is 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.

In this case, you may notice fogging on either the inside or the outside your window, and it may not be due to a having a lot of people breathing near or inside the windows. It may simply be due to the amount of water molecules in the air, which heat can cause to collect on the surface of the window if it is still cool enough that the water does not become vapor.

Surface Tension

Tiny droplets form from water since water molecules choose other water molecules as opposed to something like glass. Thus, droplets may cover an entire surface such as glass for a considerable time without ever 'soaking in' to the material. Rather than being an even sheen of water spread across the material, surface tension separates the molecules into droplets which separate from one another, almost as if in clusters. 


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. It is best not to use your hand to wipe away fog, as grease and oils from your hand may collect on the window creating more visibility issues.

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. Your A/C uses water to operate, so running the A/C will reduce the amount of moisture in the air over time.

Evaporate the Water Vapor

In some cases, you may be noticing fog collecting on the inside of your windows due to the relatively higher temperature inside which means water in the air will condense on the surfaces which are in contact with a relatively cooler temperature. By contrast, if the temperature outside is warmer and there is a significant amount of humidity, you may notice fog building up on the outside of your windows.

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.

Let’s quickly go over some things you can do to eliminate the fog from your windshield:

  • 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.

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.

A fogged up window is an easy fix. But if you have a chipped or cracked window, give Crystal Clear View Glass a call at 602-405-9151. We help with your insurance claim or take an out of pocket project, either way we will get you back on road seeing clearly again!

Whether working with businesses or individuals, everyone at Crystal Clear View Glass puts people first. It starts with Brandon, who hires self-directed professionals and empowers them to make a difference in every interaction. That culture is pervasive, from the team member who answers your call to the installer who expertly completes your glass project.