Why Do Car Windows Fog Up From the Inside? | RapidSmartFacts

You’re driving on a cool, damp day. Suddenly your windshield begins to cloud

inside. Why does this fog form on the inside, not outside?

Fog on glass is just tiny water droplets. Droplets form when humid air hits

a cooler surface. Inside the car, you are the source of humidity. Breathing,

damp clothes, and wet shoes add moisture. When moist air touches the cold

window, it cools. Cooler air holds less water vapor. This extra water

condenses into droplets on the inside surface. That’s why turning on your defroster

helps. Warm, dry air from vents heats the glass.