“In Chicago . . . my car windshield is often covered with frost first thing in the morning . . . but no frost accumulates if I leave the car under a carport or in the garage. So it seems that it must fall from the sky yet there’s nothing perceptible that’s falling. Does it fall? “
Does frost falls from the sky?
Every night, as soon as the sun sets, the surface of Earth starts to cool. Moisture hanging in the air as vapor begins to condense or change to liquid. That night, if the temperature goes below freezing, this moisture passes directly from a gaseous to a solid state. Ice crystals appear on your lawn.
This frost fall doesn’t come out from the sky. It goes directly from the air onto the grass or your car or anything else sitting out in the open. But when you put your car in the garage or under a carport something else happens.
Like everything else on Earth, your carport or garage emits infrared or heat radiation. It keeps the air around it warmer so frost doesn’t collect. Maybe you’ve been walking to work some fall morning admiring the frost-covered lawns and you’ve noticed that the ground around a tree is frost-free. The same thing is happening there. The tree’s own heat radiation keeps the “frost fall” from forming.