How to Defog a Windshield
Why do windshields fog up? Because their temperature is below the dew point of surrounding air. Dew point is a combination of temperature and humidity.
To fix it: Raise the windshield temperature while lowering the dew point of surrounding air, without fiddling with knobs, spilling coffee and scaring your passengers who would appreciate a calm defogging of the windshield.
#1 Crack the windows
If the air outside is less humid than inside (most common, as body heat and your breath fog up the interior), cracking a window will lower the dew point inside the car and cause the fog to dissipate.
#2 Blast the heat
Obviously, this will raise the temperature of your windshield, although this only works if your car is sufficiently warmed up. It’s also the only way to get rid of condensation on the outside of the glass, common in humid climates and/or on cool mornings. Use the defrost/defog vent and crank it to high.
#3 Turn ON the AC
Little known fact: your car’s AC is also a dehumidifier. So even while using warm air to defrost the windshield, make sure the AC is turned on so your car will dehumidify the air before it warms it. In most cars, the AC will automatically be turned on while using the defrost/defog vent setting. And turn off the recirculate feature to help replace interior air with less humid air from outside.
Crack window, blast heat, turn on AC… done. Oh, and resist wiping the window with your hands unless it’s an emergency, because it’ll leave streaks all over your glass later. NOTE: in winter situations where fog is quickly forming while your driving 75mph on the interstate, the quick/uncomfortable solution is to lower all the windows and replace inside air fast. Cold, but effective.
If you found these tips useful, share them with your friends—so they can deal with their own foggin’ problems.