How Does Lake-Effect Snow Form?

Locations along the southern and eastern shores of the Great Lakes are famous for the tremendous amounts of snow they receive. When the snow starts to fall, people start tossing out the phrase "lake-effect snow."

Primarily a late-autumn and early-winter phenomena, lake-effect snowstorms are instigated by the movement of cold, arctic air over the relatively warm waters of the Great Lakes. Ideally, the temperature difference between the lake water and the overrunning air should be at least 20 degrees in order for clouds and subsequent snow to form.

As the cool air crosses the waters of Great Lakes, the lowest levels of the atmosphere begin to warm and pick up moisture. This newly warmed layer is lighter than the cold air above it, so it starts to rise. As the modified air continues to climb higher into the atmosphere, it encounters much colder temperatures. This cooler air forces the moisture to condense into water droplets and ice crystals, forming clouds. After this process repeats itself a number of times, the cloud becomes weighed down and is forced to precipitate in the form of snow.

Out of all the meteorological factors that determine snowfall intensity, the most important might be the direction of the wind. If the wind direction is running perpendicular across the lake, there won`t be ample time for clouds to develop. However, if the wind runs parallel to the length of the lake, clouds should form without a hitch. The longer the cold air travels over the lake, the more moisture it is able to accumulate, which in turn leads to greater snowfall totals.

The highest annual lake-effect snowfall totals are found across the U.P. of Michigan, northwestern Pennsylvania and the far southwestern and upstate sections of New York. In these locations, which are all located along the southern or eastern shores of the Great Lakes, recording more than 100 inches of snow in a winter season is a common occurrence.

Image: Bands of lake-effect snow are flowing off lakes Superior and Michigan, bringing snow to Michigan, in the NASA satellite file image. (NASA, Wikimedia Commons)