Why Do Windshields Fog Up (and How to Clear Them)?

Car windows fog up when the temperatures inside and outside the car are very different.  In other words, in the summer it happens when a windshield, cooled by cabin A/C, meets hot outside air.  In the winter, by contrast, it happens when a windshield, heated by the cabin heater, meets the cold outside air.  Therefore, the methods for clearing a foggy windshield differ by the season.

SUMMER FOG:

If your windows are foggy in the summer, turn down the A/C and/or open your windows to better match the interior temperature to the exterior.  Next, since the fog in summer is on the outside of the windshield, run your windshield wipers at the lowest setting until the fog clears.

WINTER FOG:

If your foggy windshield occurs during the wintertime, remove any frost outside the window.  Next, turn off your recirculate button to pull in the colder outside air.   Using your defrost vent, turn your car fan on high and turn the temperature down to “cold”.  This will direct cool air directly towards your windshield and help the temperature match the outdoor air.

FOG PREVENTION:

Sometimes we bring contamination into our vehicle without even realizing it.  Many products we take for granted as being “for cars” actually add chemicals to the air which coat our glass and enhance fogging:.  So, to reduce fogging, consider eliminating the following:

• Hanging car air fresheners
• Car interior cleaners which coat/protect vinyl/leather
• Vaping in the car

Beside chemical contamination, fog is more pronounced with moisture.  Here are some remedies to remove moisture from inside your cabin.

• If possible, when parked keep your windows cracked open at the top to allow outside air to get into the car.
• Remove wet items from your car—wet clothes, umbrellas—even unconsumed food and drinks
• Do you know those silica packets that come in pill bottles, and many food packages? They’re there to remove moisture.  Make your own “car version” of these packets by filling a sock or two with silica cat litter and placing them on your dash.
• Create a moisture barrier by applying a light coating of shaving foam to your windshield and wiping it with a clean, dry cloth..

Other solutions

• Keep your windows clean and streak-free with an ammonia-based or vinegar-based cleaner
• Always use a clean, dry cloth to wipe windows

WARNING:  Never drive with a foggy window—poor visibility strongly increases the occurrence of crashes

LMK 2019