Why Does the Windshield in a Car Fog Up?

by S.F. Heron
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Temperature differences between the inside of the car and outside create moisture that can build up on the inside of the windshield. Add increased humidity levels, and it's hard to drive until the glass clears. Foggy windows occur from condensation when moist, warm air meets cool air.

Whether it's your breath inside the vehicle or a damp, rainy day, this provides adequate condensation to steam the much cooler windows of your car. Switch your car's defroster to fresh air to help clear your windshield quickly. Air conditioning removes moisture from the air. Fog requires warm, moist air to occur. Sometimes, even the breath of the people inside the car can generate enough warm air to fog the windows.

Dirty Windshield

A dirty windshield can cause windows to fog more frequently. When dust and fingerprints mar the inside surface of the windshield, moisture particles from condensation adhere more readily to this dirty surface. A foggy windshield resulting from a dirty interior glass surface is difficult to clear with the car defroster. Clean the inside of your window with a high-quality glass cleaner and dry paper towels.

Remember that the inside of your car holds moisture because it's a sealed compartment. The moisture builds up and condenses into fine water droplets on your windshield. Keeping the window clean allows your defroster to work properly, to allow a clear line of sight through the front window of the car.

Mechanical Problems

Most vehicles built after 2000 have cabin filters to help regulate air quality inside the vehicle. These filters catch pollen, dust, particles and other contaminants to prevent their entry into the passenger area of your vehicle. Air conditioning and heat filter through these cabin filters as well. Dirty cabin filters can result in more dust inside your vehicle and can limit the effectiveness of your defroster.

Consider the inside of the car as you would a building, subjected to indoor air quality issues without proper ventilation. Cabin filters function to provide clean air from your car's heat and A/C. When these filters are dirty, the poor indoor air quality inside your vehicle allows more condensation to form on your windshield. Replacement of the cabin filter often reduces fog buildup on the windshield. These filters are often located behind the glove compartment and require changing every 1 to 2 years. Check owner's manuals, or contact your mechanic if there's a musty smell or you're having trouble cleaning a foggy windshield.

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