Car windows fog up because of condensation, which occurs when warm, moist air inside the car comes into contact with the cooler surface of the glass. The temperature difference causes the water vapor in the air to turn into tiny liquid droplets on the window. This is most common in humid or rainy weather and during colder months when the interior of the car is significantly warmer than the outside air.
The science behind it is the dew point—the temperature at which air becomes saturated with moisture and can no longer hold it as vapor. When the window's surface temperature is at or below the dew point of the air touching it, fog forms. The primary sources of the moisture are your own breath, wet clothing, or damp items inside the vehicle.
| Condition | Interior Temperature | Exterior Temperature | Relative Humidity | Likelihood of Fogging |
|---|
| Cold Rainy Day | 70°F (21°C) | 40°F (4°C) | 85% | Very High |
| Winter Morning | 68°F (20°C) | 25°F (-4°C) | 70% | High |
| Humid Summer Night | 75°F (24°C) | 65°F (18°C) | 95% | Moderate (on outside) |
| Dry, Sunny Day | 72°F (22°C) | 72°F (22°C) | 40% | Very Low |
To clear fog quickly, you need to balance the temperature and humidity. Turn on your defroster setting with the A/C compressor engaged. The A/C acts as a dehumidifier, removing moisture from the air before it is warmed and blown onto the windows. For a quick fix on a cold day, cracking a window open can equalize the humidity levels faster. Preventing fog involves keeping your windows clean, as dirt provides more nucleation points for droplets to form, and using an anti-fog treatment can create a protective coating that beads the moisture.
