STEWART KNOWITALL: How and why does snow fall in the winter?

For snow to fall, clouds must form. The air always contains moisture, which is really water in the form of gas or vapor. Clouds are made up of water vapor that has changed to liquid or ice form. For clouds to form, the air must rise. Air can rise for many reasons. Most often, warm and cold air coming together at a weather front will cause lift. Near the Great Lakes, air rises in a different way. A large body of water will keep the heat it received from the sun during the summer. In winter, cold air will blow over the warm water, causing the air to rise.

As air rises, it cools. Cooling the air makes it more likely that the moisture in the air will turn to liquid. If it is cold enough, ice crystals will form. The ice crystals that form are very, very tiny. The ice crystals will begin to fall down inside the cloud. As the ice crystals fall, three things happen to them that cause them to become snowflakes, which are much larger than ice crystals. First, liquid drops can freeze onto the ice crystals. Second, ice crystals can collide with and stick to other ice crystals. Finally, ice crystals can break apart as they hit liquid drops and other ice crystals. These broken ice crystals can then grow on their own. This causes many, many snowflakes to form, resulting in snowfall.

Marty Baxter

Associate Professor of Meteorology at Central Michigan University

He has held forecasting, research, and teaching internships at Weather or Not, Inc. in Shawnee, KS, the Hydrometeorological Prediction Center in Washington, D.C., and the Cooperative Program for Operational Meteorology, Education, and Training in Boulder, CO. His research has focused on improving our understanding of precipitation systems. In particular, he has examined the climatology of snow to liquid ratio and methods for forecasting this quantity.