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

You're driving. Suddenly, your windshield fogs up from the inside.

Why does fog form on the inside, not outside? Fog on glass

is just tiny water droplets. Those droplets form when humid air hits a cooler

surface. Inside the car, you’re the source of humidity—breathing, damp clothes,

even wet shoes. When that moist air touches the colder window, it cools down.

Cooler air can’t hold as much water vapor. The extra water condenses into droplets

on the inside surface. That’s why turning on the defroster and fan helps.

Warm, dry air from vents heats the glass and dries the air.