It usually happens on a morning when you're already behind schedule: As you approach your car you can see that the windows are covered with frost or condensation, and you have to spend time — almost always time you don't have — clearing it off to drive safely.
In another of life's mysteries, at times only some of a car's outside windows may have frost or condensation, not all. What's up with that?
Related: Why Do My Car's Windows Fog Up?
Condensation on the outside of a car is caused when warm, moist air (often present in early morning hours) condenses when it hits the car's colder window glass. That's also why on warm, humid days condensation may form at the base of a windshield, where outside air encounters cooled air from the car's air-conditioning system.
Mats Selen, a University of Illinois physics professor, said that when warm, moist air is cooled, it eventually gets to a temperature at which the air can no longer support all the water, which starts to condense. That's the dew point.
"When relative humidity is 100 percent, it means that if the temperature goes down any more, then the water has to come out somewhere," Selen said. "On a cool morning, when there's dew on the ground or fog, that's because you have warm, moist air that comes in contact with the cool ground" — or condensation on a car that's been sitting outside.
When it's cold enough, the condensation turns to frost.
Why would only some car windows get condensation? Selen suggested that wind could be a factor, but another focuses on whether the windows are vertical (like side windows) or at an angle (like a windshield or the rear window on some cars). The theory is slanted windows are more likely to collect condensation because they provide a more horizontal landing spot.
Cars.com managing editor David Thomas often parks test cars in his driveway and has noticed that significantly more condensation forms on the side windows facing south. Checking historical weather for his town, Thomas noted that the wind was headed in a south-southeastly direction during the early morning hours on these occasions.
Those who park their vehicles outside are basically at the mercy of Mother Nature as to temperature, humidity, dew point and whether the combination results in condensation. Selen, though, said one way to possibly reduce the amount of moisture that collects on your windows is to coat them with a water-repellent substance such as Rain-X or Aquapel, which both cause water to bead and roll off the glass. No guarantees, though.
Cars.com photos by David Thomas
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