Should You Use Cold or Hot Air to Defog the Windshield?

[DeLucille]

Using hot air to defog the windshield is more effective than using cold air. The reasons for windshield fogging include the following: The windshield temperature is lower than the dew point: When the windshield temperature is lower than the dew point at a given air humidity, condensation forms on the windshield surface, which is the common fogging phenomenon observed on the front windshield. A temperature difference exists on both sides of the windshield: When there is a temperature difference between the two sides of the windshield, and the saturated vapor pressure on the cooler side is lower than the surrounding vapor pressure, moisture gathers on the glass surface, forming tiny water droplets that create fog. 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.