Lake-effect clouds off Lakes Superior and Michigan; satellite image taken December 5, 2000, courtesy of NASA.
Lake effect snow is produced in the winter when cold winds move across long expanses of warmer lake water, picking up water vapor which freezes and is deposited on the lee shores. This effect is enhanced when the moving air mass is uplifted by the orographic effect of higher elevations on the downwind shores. This uplifting can produce narrow, but very intense bands of precipitation, which deposit at a rate of many inches of snow per hour. The areas affected by lake effect snow are called snowbelts.
Cold winds in the winter typically prevail from the west-southwest to the northwest in the North Temperate Zone, producing the most dramatic lake effect snow falls on the northeast to south east shores of the Great Lakes. This lake effect produces a significant difference between the snow fall on the eastern and western shores of the Great Lakes. If the air temperature is not low enough to keep the precipitation frozen, it falls as lake effect rain. Lake effect snows at the southeastern side of Lake Ontario frequently set the daily records for snowfall in the United States. Lake Erie also produces a similar effect for a zone stretching from the eastern suburbs of Cleveland to Buffalo, but Lake Erie often freezes due to its shallowness, and ice cover stops the lake effect. The southern and southeastern sides of the Great Salt Lake also receive significant lake effect snow, but it is much more severe over the Great Lakes.
Lake effect of extremely cold air over still warm water in early winter can produce thunder snow, a snow storm with intense lightning and thunder.
Even when precipitation is not produced, any time cold air passes over warmer water it produces cloud cover. Since the prevailing winds over the Great Lakes are from the northwest, and they tend to be colder than the water for much of the winter, the southeastern shores of the lakes are almost constantly overcast, leading to the use of the term The Great Gray Funk as a synonym for winter. These areas have a high rate of seasonal affective disorder, a type of psychological depression thought to be caused by lack of light.
Similar snowfall can occur near large inland bays, where it is known as Bay effect snow. Note that Nor'easters are ocean effect storms produced by offshore lows on an Eastern coast, which bring moist, unstable air across land areas, often opposite to the usual prevailing winds.