Battling Foggy Windows

031_fogwindshield_v2Cold mornings are notorious for creating foggy windows. During the winter dirty streaks form on the windshield from mittens attempting to give relief to the squinting driver, but instead leave behind more debris that attracts and adheres the accumulation of fog. Summer is also frustrating when the moisture settles on the outside of the windshield just out of the drivers reach.

Thankfully there’s a solution for foggy windshields that works.


The key is making sure the dew point is never reached inside of your car. The dew point is that magic moment when, based on humidity and barometric pressure, water vapor in the air cools to the point that it clings in the form of droplets. In other words, the moisture in the air clumps together and forms a drop of water.

When driving to grandma’s house for the winter holidays, people love to crank the heat in their car so they can take off their jackets and sing songs, which create the ideal circumstances for foggy windows. Let’s look at the science of it.

The warm moist exhaling breath from those singing is pushed into the car. The heat in the car is turned down to compensate for the hot air and everyone moving to the beat. In the meantime, the windshield is being beaten from the outside by chilly winds and cold snowflakes, creating a perimeter of colder air around its inside surface. The cold windshield chills the moist interior air, which clumps together and adheres to the windshield in the form of fog.

The driver now has two choices: bring dryer air into the car to reduce the moisture; or, heat up the windshield to evaporate the fog.

DRY AIR can be vented into the car by shifting from recirculating air to outside air at a push of a button. The dryer air will automatically absorb some of the moisture, albeit slowly.

DEFROSTING the windshield applies heat to the glass and evaporates the moisture. Once the temperature of the glass pushes the cold to the outside of the windshield, moisture will only be able to accumulate outside of the car, which is easily wiped away by windshield wipers. Turning on the AC at the same time will speed the drying results.


During air conditioning season many people crank the cold air inside the car. After some time the windshield gets cold enough that it forms a cool barrier on both sides of the glass. On humid days, the cold glass reduces the humidity down to the dew point and collects the moisture in the form of fog on the outside of the windshield.

RUNNING WIPERBLADES is the easiest and fastest way to remove the fog, as the blades collect all the moisture and send it to the side of the glass. Unfortunately, the remaining windshield that’s not reached by the blades continues to build up fog until the droplets get big enough for water drops to run down the windshield.

RAISING THE AIRCONDITIONING TEMPERATURE to something a little warmer will slow the glass from reaching the dew point. For even faster results, turning off the air conditioner and opening the windows for a time will cause all of the fog to evaporate within minutes.

The fastest way to correct for fog is to make sure the dew point isn’t reached inside of the car during the winter and on the outside during the summer.

Copyright © 2016 Hi-Tech Automotive Specialists, Inc.

About CJ Powers

CJ is an author and speaker.
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