When is the defogger in a car is used?

Answer 1

When you push the button. They are not automatically engaged on most vehicles but there are newer models that have integrated moisture sensors that can detect fogging of the rear window. The system is designed so that when fog builds on the inside or outside of the rear window (typically but also side mirrors) the little lines that run through the glass are actually heat coils that get warm as electricity passes through them thereby "burning" off the moisture from the window, can also be engaged to warm the glass to remove ice in such climates that produce ice. The front windshield and side windows are also given a form of defogger or defroster, they are air driven via your vehicles heat and a/c system. when there is high humidity and the windows reach the scientific dewpoint the windows will get the moisture build up aka fog, the resulting combat to this is to engage your defogger/defroster air system, it will either blow the cold air from your hvac or the warm air from your hvac across the windshield and the mirror area of your side windows.

the most common days that the defogger will be used is when it is rainy, usually in the afternoon or evening when the ambient exterior temperature has decreased but the humidity has remained constant or increased