Lake-Effect Snow

In some parts of the world, the heaviest snowfalls do not come from large winter storms but from a much more local process known as lake-effect snow. It is especially common near the Great Lakes of North America, where towns on the eastern and southern shores can receive several meters of snow in a single winter, while places only a short distance away receive far less.

Lake-effect snow forms when very cold air moves across a large body of water that has not yet frozen. In late autumn and early winter, the lakes are still relatively warm, because water loses heat much more slowly than land does. As the cold air passes over a lake, it picks up heat and moisture from the water's surface. The warmer, moister air rises, cools, and forms clouds. When the air reaches land on the other side, it slows down, and the clouds release their moisture as snow, often in narrow bands only a few kilometers wide.

Several factors determine how much snow falls. The greater the difference in temperature between the lake and the air, the more intense the snowfall tends to be. The distance the wind travels over open water also matters: a longer path allows the air to collect more moisture. Once a lake freezes over, however, lake-effect snow largely stops, since the ice cuts off the supply of heat and moisture.