Why Do Car Windows Fog Up in Cold Weather?

Car windows fog up in cold weather because water vapor condenses on their inside surface, according to the University of Illinois Physics Department. People breathing can sometimes be the main source of water vapor inside a car.

Causes

Condensation occurs when the dew point of the air inside a car is warmer and more humid than the air outside the car window. When warm water vapor touches the cooler glass, the vapor cools down and becomes water.

Prevention

In cold weather, you can defog the car windows by running the air conditioner to dehumidify the air, running the heater to warm the condensate to turn the water into vapor and venting the stream toward the glass.

Tip

To keep windows from fogging up, lower the temperature setting. Do not keep the fan running on "recirculate," which would only circulate warmed air through the cabin and keep the windows foggy.

References

• University of Illinois Physics Department: Car Windows Fogging Up
• DC Physics: Avoiding Frost & Fog on Windshields