The pearly drops of dew in the grass and the feathery fingers of frost on the window pane are formed in very much the same way. The only difference in the recipe is the temperature. Both are formed from gaseous water vapor which the air has gathered during the warmer sunlit hours. As this warm air cools, it must give up some of this vapor.

If the early morning is too warm to freeze water the surplus vapor is turned into liquid dew drops. If the temperature is below freezing, the tiny particles of vapor will turn directly into fragments of frosty ice.