Ask A Scientist© Environmental Science Archive

Author:      janette l gubala
What makes it snow?

Response #:  1 of 1
Author:      grant l
Snow originates in clouds that are below freezing, but it is not simply frozen 
water.  The formation of snow requires that dust or other particles be present 
in the clouds.  Molecules of water vapor adhere directly to the particles, 
eventually forming snowflakes.  When the snowflakes are heavy enough, they 
fall to the ground.  As the snowflakes fall through the clouds, sometimes ice 
crystals break off the snowflakes; these ice crystals form nucleation points 
for more snowflakes, so that one dust particle may be responsible for numerous 
snowflakes.

Snowflakes are shaped differently depending on temperature, humidity, and 
barometric pressure.  The larger snowflakes form under high humidity and 
relatively high temperatures, since, when the temperature is near freezing or 
slightly above freezing, the snowflakes become wet and adhere to one
another.  Snowflakes several inches across can be formed in this manner.

The above information is taken from, "It's Raining Frogs and Fishes," by Jerry 
Dennis, Harper Perennial, 1993.