Why Does the Outside Fog Up When the Air Conditioning Is On?
2 Answers
The reason the outside fogs up when the air conditioning is on is that the temperature inside the car is lower than the temperature outside. Using the Air Conditioner to Defog: This is the simplest method. Just turn on the front window defogging mode, set the fan speed to maximum, turn on the A/C switch, and adjust the temperature to the lowest setting. The fog on the glass will disappear shortly. Additionally, some car models have heated rear windshields and side mirrors. Simply click the switch to activate them, and the fog will clear in about a minute. Using Anti-Fog Spray: To use, simply spray it evenly on the glass surface and then wipe it clean with a dry towel. Its principle is to use surfactants to form a protective film on the glass surface, altering the surface tension of water molecules so that fog droplets coalesce into larger droplets, thus not obstructing the view. This method is also suitable for other car windows, especially the side mirrors.
I often notice fog forming on the outside of my car when using the air conditioning in summer, especially with water droplets appearing at the bottom of the windshield or on the hood. This happens because the cold air from the AC meets the warm, humid outside air—like after rain or in damp areas—causing the moisture to condense into fog on the cooler car surfaces. It's the same principle as fog forming when you open a fridge door—a purely physical phenomenon, not a car malfunction. To reduce this effect, try turning up the AC temperature slightly instead of keeping it at the lowest setting, or adjust the vents to blow upward rather than directly outward. If heavy fog obstructs visibility, simply use the wipers to clear it quickly. Regularly cleaning the car's exterior after parking can also help prevent fog buildup. While not a serious issue, maintaining good ventilation is important.
