In cold weather, the air inside reaches a high humidity while you are driving and keeping the vehicle comfortably warm. If there are a few people inside, the humidity can become very high.

When you park and leave the vehicle, the humidity remains inside. The inside air was at a relatively low humidity. However, when the inside air drops from warm to cold, the relative humidity goes up.

As the inside air cools it is less able to hold the same amount of moisture as when you were driving and the air was warmer. As the air inside cools down to ambient temperatures, the moisture in the cooling air condenses on cold surfaces. It condenses especially well on the smooth surface of glass.

This is normal and will happen to greater degrees with more people in the vehicle and when the outside air is especially cold.

You can reduce the amount of frost that forms on the windows by airing-out the vehicle just before you leave it.

Turn off the heater, open a few windows, and allow the outside cold, dry air to replace the inside warm, moist air. This should take only a minute with a few windows open.