Should You Use Cold or Hot Air to Defog Windows?

[DelSimon]

Using hot air to defog windows is more effective than using cold air. There are several reasons why car windows fog up: 1. The window temperature is lower than the dew point: When the window temperature is below the dew point at a given air humidity, condensation forms on the window surface, which is the common fogging phenomenon seen on the windshield. 2. A temperature difference exists between the two sides of the window: When there is a temperature difference between the two sides of the window, and the saturated vapor pressure on the colder surface is lower than the surrounding environment's vapor pressure, moisture gathers on the glass surface, forming tiny water droplets that create fog. 3. The dew point is above zero degrees Celsius: If the dew point is above zero degrees Celsius, fogging occurs; if it is below zero degrees Celsius, frost forms instead.