Why do car windows fog up more easily in cold weather?
The Real Answer
Car windows fog more easily in cold weather because the glass becomes cold enough for moisture in the cabin air to condense on it. Warm air inside the car can hold water vapor from breathing, wet clothing, snow, rain, and other sources. When that moist air touches colder glass and the surface is at or below the air’s dew point, some of the water vapor turns into tiny liquid droplets.
Those droplets scatter light instead of letting it pass cleanly through the glass, producing the cloudy layer we recognize as fog. Cold weather makes the problem especially common because the temperature difference between the warm cabin and the windows can be large.
This is also why the defroster helps. Warming the glass, lowering the cabin humidity, or bringing in drier outside air can move the window surface away from the conditions that allow condensation. Interior window fog is therefore a condensation problem; frost or ice involves water freezing and is a different process.
You get into a cold car, breathe twice, and suddenly the windshield looks like someone installed a privacy setting. Then you wipe one little circle with your sleeve and drive around peering through it because apparently seeing eight inches of the road feels sufficient.
Why People Ask This
People ask because window fog can appear quickly after people get into a cold car, even when the glass looked perfectly clear moments earlier. The combination of warm passengers, cold windows, and disappearing visibility makes the effect both obvious and inconvenient enough to invite the question.