What weather phenomenon occurs when cold air passes over a warm lake, producing heavy snow?

When cold air passes over a warmer lake and produces heavy snow, the phenomenon is called lake-effect snow. The lake warms the lowest layer of the moving air and adds water vapor to it. That moist air rises into the colder atmosphere, where the vapor condenses, freezes, and falls as snow on the downwind shore.

The process is strongest when a long stretch of unfrozen water lies beneath a very cold air mass. Meteorologists call that distance the fetch. Longer fetches give the air more time to collect heat and moisture, helping narrow snow bands grow intense. Hills or higher terrain on the downwind side can force the air upward and increase snowfall further.

The Great Lakes are the best-known setting, especially around western New York, Ontario, Michigan, and Ohio. Areas receiving frequent lake-effect snow are known as snowbelts. The result can be highly localized: one town may receive several feet while a nearby area gets little. If the air is not cold enough, the same process produces lake-effect rain instead.