Lake-effect snow is produced in the winter when cold, Arctic winds move across long expanses of warmer lake water, providing energy and picking up water vapor which freezes and is deposited on the lee shores. The same effect over bodies of salt water is called ocean effect snow, sea effect snow, or even bay effect snow. The 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 each hour and often bringing copious snowfall totals. The areas affected by lake-effect snow are called snowbelts. This effect occurs in many locations throughout the world, but is best known in the populated areas of the Great Lakes of North America.
If the air temperature is not low enough to keep the precipitation frozen, it falls as lake-effect rain. In order for lake-effect rain or snow to form, the air moving across the lake must be significantly cooler than the surface air (which is likely to be near the temperature of the water surface). Specifically, the air temperature at the altitude where the air pressure is 850 millibars (roughly 1.5 vertical kilometers) should be 13°C lower than the temperature of the air at the surface. Lake-effect occurring when the air at 850 millibars is much colder than the water surface can produce thundersnow, snow showers accompanied by lightning and thunder (due to the larger amount of energy available from the increased instability).
There are several key ingredients that are required to form lake effect precipitation, and which determine its characteristics: instability, fetch, wind shear, upstream moisture, upwind lakes, synoptic (large)-scale forcing, orography/topography, and snow or ice cover.
Speed shear unlike directional shear is less critical but it should be relatively uniform. The wind speed difference between the surface and 700mb level should be no greater than so as to prevent the upper portions of the band from shearing off. However assuming the surface to 700mb winds are uniform, a faster overall velocity will work to transport moisture quicker from the water and the band will travel much further inland.
Even when precipitation is not produced, cold air passing over warmer water may produce cloud cover. Fast moving mid latitude cyclones, known as Alberta clippers often cross the Great Lakes. After the passage of a cold front, winds tend to switch to northwest, and a frequent pattern is for a long lasting low to form over the Canadian Maritimes which may pull cold northwestern air across the Great Lakes for a week or more. Since the prevailing winter winds 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 allegedly contain populations that suffer from high rates of seasonal affective disorder, a type of psychological depression thought to be caused by lack of light.
Cold winds in the winter typically prevail from the northwest in the Great Lakes region, producing the most dramatic lake-effect snow falls on the east to south 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.
Lake-effect snows on the Tug Hill Plateau (east of Lake Ontario) can frequently set the daily records for snowfall in the United States. Syracuse, New York is directly south of the Tug Hill Plateau and receives significant lake-effect snow from Lake Ontario, averaging of snow a year, which is enough snowfall to often be considered one of the "snowiest" large cities in America. The communities of Redfield in Oswego County and Montague and North Osceola in Lewis County, all on the Tug Hill Plateau, average over of snow a winter. In February, 2007, a prolonged lake-effect snow event left of snow on the Tug Hill Plateau.
Small amounts of lake-effect snow from the Finger Lakes occurs in upstate New York as well, until those lakes freeze over. The Appalachian Mountains and Atlantic Ocean largely shield New York City and Philadelphia from picking up any lake-effect snow; snow there tends to come from storm systems mixing with cold weather.
Lake Erie produces a similar effect for a zone stretching from the eastern suburbs of Cleveland to Erie to Buffalo. Remnants of lake-effect snows from Lake Erie have been observed to reach as far as Garrett County in western Maryland. The Canadian lake effect belt spans from Port Stanley in the west, Brantford in the north, Niagara-on-the-Lake to the northeast and Fort Erie to the south, with the cities of Nanticoke, Dunnville, Port Colborne and Fort Erie seeing the greatest accumulations. Lake Erie has the distinction of being the only great lake capable of completely freezing over during the winter due to its relative shallowness. Once frozen, the resulting ice cover alleviates lake-effect snow downwind of the lake.
The biggest snowbelt in the United States would be the snowbelt in the Upper Peninsula (UP) of Michigan, near the cities of Houghton, Marquette, and Munising. These areas average over of snow a season. For comparison, on the western shore, Duluth, Minnesota receives per season. Lake Superior and Lake Huron rarely freeze due to their size and depth; lake-effect snow can fall continually in the Upper Peninsula and the Ontario, Canada snowbelts during the winter months. There are 4 distinct Lake Huron snow belts that exist in Ontario, the first spans from Sarnia to Grand Bend, with cities as far south as London sometimes receiving significant accumulations. The second spans from Grand Bend to Owen Sound and north to Tobermory with significant snows often affecting regions as far south as Arthur, Orangeville and Caledon. Under extremely unstable conditions significant snows may reach Kitchener, Guelph and western portions of Halton and Peel Regions. The third distinct belt spans from Owen Sound in the West to Cannington in the East, and Gravenhurst to the North, with significant snows often affecting the cities of Barrie and Orillia and to a lesser degree northern portions of York Region, such as Newmarket, Aurora and King City. The fourth belt spans from Noelville which is 50 km south of Sudbury to Gravenhurst, and as far East as Ont. Route 62, with strong streamers often reaching into Algonquin Park. Lake Superior has its own series of Canadian snow belts, the first spanning from Wawa to Sault Ste. Marie with streamers travelling as far east as Hwy. 129. The second belt spans from Marathon to Wawa with Pukaskwa National Park receiving the majority of the precipitation. Sault Ste. Marie.
Western Michigan, western Northern Lower Michigan, and Northern Indiana can get heavy lake-effect snows as winds pass over Lake Michigan and deposit snows over Traverse City, Grand Rapids, Kalamazoo, and South Bend, but these snows abate significantly before Lansing or Fort Wayne, Indiana.
Cities such as Toronto, Hamilton, Detroit, Milwaukee, and Chicago frequently miss out on lake-effect events because they are not on the leeward shore of a lake during the predominant northwest wind seen during many lake effect events. However Toronto and Hamilton are close enough to Georgian Bay and Lake Huron that they receive small amounts of lake-effect snow each winter with a typical westerly or north-westerly wind. Nevertheless, both cities have received significant accumulations resulting from rare easterly and south-easterly winds, usually associated with northern potions of winter cyclones and sometimes southern portions of anti-cyclones that can generate snow squalls from Lake Ontario. Even so, a less frequent easterly or northeasterly wind can deposit heavy snows on Chicago or Milwaukee much as a northwest/westerly wind does for the opposite side of Lake Michigan.
The southern and southeastern sides of the Great Salt Lake receive significant lake-effect snow. Since the Great Salt Lake never freezes, the lake-effect can affect the weather along the Wasatch Front year around. The lake-effect largely contributes to the 55-80 inch (140-200 cm) annual snowfall amounts recorded south and east of the lake, and in average snowfall reaching 500 inches (1,270 cm) in the Wasatch Range. The snow, which is often very light and dry due to the desert climate, is referred to as "The Greatest Snow on Earth" in the mountains. Lake-effect snow contributes to approximately 6-8 snowfalls per year in Salt Lake City, with approximately 10% of the city's precipitation being contributed by the phenomenon.
The Finger Lakes of New York also are long enough for lake-effect precipitation. The twin cities of Sherman, Texas and Denison, Texas are also known to have experienced lake-effect snow from Lake Texoma in rare instances.
The West Coast occasionally experiences ocean-effect showers, usually in the form of rain at lower elevations south of about the mouth of the Columbia River. These occur whenever an Arctic air mass from western Canada is drawn westward out over the Pacific Ocean, typically by way of the Fraser Valley, returning shoreward around a center of low pressure.
Other small lakes such as Lake Athabasca in Northern Saskatchewan and Lake Nipigon in Northwestern Ontario also produce early season lake effect snows. The Smallwood Reservoir, a man made lake located in Labrador has on occasion also generated lake effect snow.
A phenomenon similar to lake-effect snow may also occur in other countries, near large lakes or large sea areas. One such example is the Aegean Sea in Greece, where cold northeast winds known as the boreas combined with the sea moisture can produce very heavy snowfalls over Athens (particularly across the mountainous northern suburbs of the city), the island of Euboea and easternmost Peloponese. These intense systems can have a duration of 2 to 5 days and result to a snowcover of 40 inches (101.6 cm) or more. One such system, that occurred between January 4 and January 7, 2002, wreaked havoc across that entire area, essentially shutting down Athens.
Similar effects also occur in the regions of the Black Sea in Georgia and Turkey or the Adriatic Sea and Italy. The snowfall in the eastern regions of the Black Sea is amplified by the orographic effect of the nearby Caucasus Mountains, often resulting in snowfall of several meters, especially at higher elevations. In Northern Europe, cold, dry airmasses from Sweden can blow over the Baltic Sea and cause heavy snow squalls on areas of the southern and eastern coasts.
In the United Kingdom, easterly winds bringing cold Continental air across the North Sea can lead to a similar phenomenon. Locally it is also known as "lake-effect snow" despite the snow coming in from the sea rather than a lake. Similarly during a north-westerly wind, snow showers can form coming in from the Liverpool Bay, coming down the Cheshire gap, causing snowfall in the West Midlands. This formation resulted in the white Christmas of 2004, in the area.
Due to the North Sea being relatively warm (around 13C at the beginning of winter, typically 10C to 6C by the end), sufficiently cold air aloft can create significant snowfalls in a relatively short period of time. The best-known example occurred in January 1987, when record-breakingly cold air (associated with an upper low) moved across the North Sea towards the UK. The end result was over a foot of snow for coastal areas, leading to communities being cut off for over a week. In recent years lake-effect snow has been much lighter, due to a lack of very cold Continental easterlies.
Warnings about lake-effect snow: