S N O W   A N D   I T S   D E F I N I T I O N S

   All forms of precipitation can be traced back to very cold conditions inside clouds, and all of the 10 kinds of winter precipitation described by the international snow classification system begin as snow.

   Snow is created when the temperature of a cloud is low enough (about 28 degrees F.) to allow the formation of crystals. It all begins with a tiny drop of water that gets bigger and bigger as water vapor condenses onto it. Eventually, the droplet gets so cold it freezes into a microscopic ice crystal. At about 5 degrees, the crystal develops the familiar hexagonal shape of 6 branched arms we identify with snowflakes, the shape kids in elementary school make out of cut paper and that you see on greeting cards. Initially these ice crystals are too tiny to see, but as they circulate in the cloud, they bump into more water vapor which condenses onto the crystal, enlarging it. Eventually, an accumulation of crystals will become too heavy to remain in the cloud and so it falls to the ground. When the air just below the cloud is cold enough that the crystals don't melt (about 30 or 31), they will reach the surface of the earth as snowflakes. When the ground is warm and the snow that touches it melts, the melting snow cools the earth and makes it possible for more falling snow to accumulate. (Make your own Winter Weather and see what mix of warm and cold air creates snow and freezing rain in the Interactive Animations, in the WHY section of the website.)

   Snow fall can be described in a number of ways, based on the amount, interval, and intensity at which it drops from a cloud to the Earth: "Flurries" are the least worrisome type of snowfall: in a flurry, a very small amount of snow falls for a short while, and little to no snow accumulates on the ground.