Should You Use Cold or Warm Air to Defog Your Car?

[DiCassidy]

There are two methods to defog: 1. Dehumidification: On one hand, it reduces the humidity in the ambient air, requiring a lower temperature to reach the saturation point of water vapor. This temperature value is referred to as the 'dew point' of the current air mass, which is solely related to the moisture content in the air. When the dew point temperature is lower than the temperature on the inner side of the windshield, it fundamentally prevents fog formation. On the other hand, dehumidified air has a greater capacity to carry water vapor. When blown onto the glass, it can 'reabsorb' or 'evaporate' the already condensed water droplets back into the air. 2. Heating: This does not refer to heating the air but rather heating the glass. By raising the inner surface temperature of the glass above the dew point, the subsequent effect is the same as dehumidification.