The lake effect
Posted by Jolan Barbançon on October 12, 2018 – News
The Great Lakes of North America form a group of 5 lakes that are the largest freshwater body in the world, covering an area of 244,000 km2. Because of their vast expanse and their geographical position, they cause a local weather phenomenon called “lake effect snow”. Prrobably the nature’s most incredible snow machine.
Each year the Lake Effect causes intense localized rain and snow events around the lakes, often reaching record amounts of precipitation. Usually short-lived, ranging from a few hours to a few days, these snowstorms and rainstorms adversely impact people living around these bodies of water.
Among the most spectacular historic events to showcase this phenomenon, was the blizzard of 1966 with snowfall of 2.6 meters in 5 days measured in the town of Oswego. In another, during January 11th and 12th 1997, two meters of snow fell in just 24 hours in Montague. The intensity and scale of such snow events has serious consequences, including dangerous blizzards, reduced visibility and deep snow closing road networks. On occasion drivers caught in the storm are buried by the snow.
The first studies of this phenomenon date back to the 1950s and are mainly led by Robert Sykes, a professor at the University of New York in Oswego. Thanks to the participation of several researchers over several years, they were able to highlight the conditions necessary to create the “Lake Effect” as well as the dynamics of the mechanisms associated with it. Today the “Lake Effect” season begins around August and September, when the water reserves of the great lakes, warmed by the summer heat, reach their peak temperatures at the same time as the air temperatures begin to decrease ahead of winter.As winter progresses, the atmosphere cools faster than the water of the large lakes, causing significant temperature differences across the air / water interface. This results in high evaporation which causes a significant increase in air moisture over the lake. At the same time polar winds from Canada flow south across the great lakes, further cooling the air and increasing the temperature difference with water, exacerbating evaporation.
In addition, the evaporation is also subjected to rapid displacement of the air masses at the water interface, increasing the vertical distribution of the humidity in the atmosphere. The combination of a large difference in temperature between the air and the water and the presence of a cold wind over the lake body, creates sufficiently rapid evaporation to saturate the lower layers of atmosphere with moisture.
Various studies show that it is around a temperature difference of 13°C between air and water that the lake effect phenomenon begins, kick starting the most incredible snow machine in nature. Cloud saturated with moisture and warmed by the lake is pressurized by the overlying mass of dense cold polar air at higher altitude. The geomorphology of the terrain and the prevailing winds during the formation of the cloud system will then be the main factors determining both the cloud’s direction of travel and the place where the snowstorm will strike.
During a journey of several hundred kilometers, the moisture saturated air cools in contact with cold air and, depending on the temperature of the latter, the moisture condenses in the form of rain, snow and ice. This eventually causes heavy precipitation over areas as much as 160 kilometers long and 30 kilometers wide. Each single line of cumulus weather cells, forms a “band” that comes mainly from the geometry of the lakes and the axis of their length.
At the beginning of the “Lake Effect” season, there is heavy rainfall, which will give way to snowstorms and ice storms as the winter progresses. In the middle of winter, as temperatures reach their minimum, the great lakes have exhausted their summer heat reserves and begin to freeze. The temperature difference between water and air decreases as does evaporation at the surface, until it stops completely around the end of January and early February. With the return of the next spring and summer, the air warms up faster than the lakes, which slowly start to accumulate heat, so that the Lake Effect cycle can begin again.
This “Lake Effect” phenomenon, so dramatic in the Great Lakes, also occurs in other parts of the world where conditions allow. They have been reported in Japan, due to cold air masses from the Arctic crossing the Sea of Japan, as well as around Lake Baikal in Russia, Balkach in Kazakhstan and, although rarer, it may also occur in France on the south coast of the English Channel. Recent studies have examined the evolution of the frequency of occurrence of these phenomena and their intensities. By comparing snowfall from 1930 on 15 sites where the “Lake Effect” is present and on 10 where it is not, there is a clear evidence of significant snowfall increases at only the Lake Effect sites.
In order to be able to support the hypothesis of an increasing impact from the “Lake Effect” phenomena, another methodology is used to characterize the precipitations which it causes. It has been found that water vapour from large lakes contains a lower ratio of a certain type of oxygen isotope when compared with water vapor from the oceans. So by studying the isotopic composition of the sedimentary layers of the Great Lakes, it is possible to trace the historical occurrences of “Lake Effect” precipitation. The analysis of the sedimentary layers of Cayuga, Owasco and Otisco lakes all show an increase in the percentage of precipitation due to the Lake Effect in more recent years.
The question then, is what causes these increases in frequency and intensity of the “Lake Effect”? The main parameters influencing the “Lake Effect” are the air temperature, the temperature of the lake water surface and the prevailing wind speed. Air temperatures do not show significant change from 1936 to 2001. By contrast with measurements of the surface temperature of the lakes (1995-2000) there is a clear increasing trend, and as a result recent observations indicate a decrease in lake ice cover in the middle of winter. Clearly water warming is having an effect on both the intensity and length of the annual “Lake Effect” season.
The various conclusions point to global warming as the cause of the increase in the surface temperature of these lakes. In the future this is likely to manifest in a continued increase in the frequency and intensity of the Lake Effect phenomena, at least for a time. When global warming eventually warms the polar cold air masses, the global difference between air and water temperature will decrease, thus gradually reducing the appearance of the “Lake Effect”.