Lake Effect Snow
White mountains of snow, heaped and sculpted by the wind, fill the streets, and clouds of puffy flakes come down so thickly that they cast a greyish shadow despite their whiteness. Bucket loaders and backhoes work feverishly to scoop the masses of frozen water out of the roadways, while cars and trucks with their headlights on at noon crawl along the roads, their tires sometimes sliding in the wet snow on the road surfaces.
This isn't a scene from some remote town on the Arctic Circle named for a little-known explorer -- instead, it could be an image from any of the areas in the United States or Canada that are affected by lake-effect snow. The "Snow Belt" of Northern Wisconsin and Upper Michigan, the four snow belts of Ontario, and the record-breaking snows that seem to strike Buffalo, New York every year are examples of lake-effect snow in action. Where lake-effect snow occurs, huge amounts of snow -- sometimes as much as 7 feet -- can fall in a matter of a few days.
Even when the amount isn't quite that spectacular, lake-effect snow can be dramatic, especially for those accustomed to more 'civilized' snowfall patterns. A lake-effect snowstorm can sweep in within a matter of an hour or two, create near white-out conditions for several hours, and then stop as suddenly as it began, leaving a deep layer of fresh, crisp snow in its wake. Alternately, it can come as an endless series of light snowfalls, day after day, which never seem to stop and eventually add up to a large amount over the course of several weeks.
Lake-effect snow is caused by a mix of cold winds and warm (relatively speaking) waters, resulting in evaporation and condensation of the water into snow over the nearby land.
The causes of lake-effect snow
A big lake is needed to cause the creation of lake-effect snow. The width of open water needed for this snow to develop is called the "fetch," and because winds crossing a lake at different angles may or may not cross a large enough 'fetch,' there are places along the same lakeshore where lake-effect snow can happen, and other places where lake-effect snow is basically impossible. This is the reason why some lakes, such as Lake Huron in the United States, have several distinct snow belts along their shores.
The basic mechanism of the lake-effect snow is fairly simple. If a cold wind passes over a wide area of considerably warmer lake waters, then the heat and moisture of the lake will evaporate upwards into these winds. Heat rises, carrying moisture with it, and the strong difference between the lake's temperature and the air temperature makes the transfer very intense. The wind becomes heated and laden with a large amount of evaporated lake water vapor.
However, as soon as it reaches land, the heat source is gone, the air cools sharply, and since cool air can't hold much moisture, the huge amounts of lake humidity condense out and are 'dumped' on the land as snow. Since the presence of strong winds is also needed to carry the moisture to land, the snow often blows and accumulates in drifts -- and may result in white-out, or nearly white-out, conditions.
Large northern lakes, such as Lake Superior in the United States or Great Slave Lake in Canada, are most prone to lake-effect snow. These lakes can produce sudden, powerful snowstorms known as snowbursts, which can drop as much as 11 inches per hour. Snowbursts typically don't last much longer than an hour, but occasionally continue for a greater amount of time and lead to record-breaking accumulations. Other areas of the world are sometimes subject to lake-effect snow, although small seas such as the Baltic or the Aegean are usually what generate "lake-effect" snow in these areas, which typically lack lakes as large as those found in North America. Athens, on the Aegean coast, can get very deep lake-effect snow when winter winds blow from the northeast. Some areas of Japan are also snow belts in the winter.