Frost on a window

This frost formation was on the outside of my truck window the early morning of Jan. 11. Rain a day or two earlier caused there to be enough humidity in the air to produce this frost.

Air temperatures must go below freezing and there should be no wind in order for this kind of frost to form. It is anybody's guess which direction the little ice crystals will go and what kind of fancy designs they will make.

Sometimes a similar frost effect will form on the inside of a glass window or door. When it is cold outside, you can open the inside door and watch the frost spread across the inside of the cold secondary glass door. The humidity from in your house allows this to happen.