Lake effect snow occurs often during the winter months. It takes place when cold Arctic air moves over the relatively warm water of the lake. During a cold outbreak, winds from the west or northwest bring the cold, dry, Arctic air over the warmer waters of the lake. As the Arctic air passes over the warmer surface of the lake, the air is heated strongly from below. Because the air is very dry, evaporation occurs into the air. The added heat and moisture near the surface makes the very unstable and turbulent. Due to the heat and moisture that the lake puts into the overlying air, the air close to the surface is very buoyant. When the warm, moist air at the surface is lifted slightly by turbulence, it is cooled as it is lifted. The temperature of the air decreases, however, it is not as rapid as in the surrounding air. This cavity of air is now warmer than the surrounding air, therefore it keeps rising. Since the air is moist, it does not cool too much before it becomes cumulus clouds. When the clouds form, water vapor is condensed into tiny droplets. These droplets are usually found as ice-crystals in a lake-effect storm. Eventually the ice-crystals form into snowflakes. As soon as this occurs, the clouds start to produce heavy snow showers over the lake and downwind of shore. This lake-effect storm can leave behind up to a foot of snow within a few hours. Reference Link: http://www.theweatherprediction.com/winterwx/lesnow/