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Lake Effect Snow is a heavy, localized snowfall in the proximity of large bodies of open water occurring when a cold air mass and accompanying winds move over warmer lake water. Lake effect snow clouds normally form in narrow bands, referred to as "streamers" which can be as much as 40 km wide and extend over 100 km inland from the lake. Within the streamer, snowfall rates can exceed 15 cm per hour.
Lake effect snow is prevalent in the Great Lakes region of North America but also occurs in several other areas.
Lake effect snow forms when a cold air mass and accompanying winds move over warmer lake waters. The lake water raises the temperature of the proximal layer of air and moisture from the lake evaporates into the air mass. The moist, warm air rises and begins to cool causing the moisture to condense, freeze into ice crystals and form clouds which are then driven onto the shore by the wind. As the ice crystals collide, they stick together to become snow flakes and, when the flakes become heavy enough, snow begins falling.
Under the right air mass and wind conditions, lake effect snow can occur virtually any time during the late autumn and winter months so long as the surface of the lake is not frozen. A frozen lake does not produce significant amounts of lake effect snow as an ice surface reduces both the temperature differential between the lake and the air mass and the available moisture. It is very unusual for the North American Great Lakes to become completely ice covered at any time during the winter. As a consequence, there is the potential for lake effect snow in the Great Lakes region throughout the entire winter season.
Lake effect snow clouds normally form in narrow bands, referred to as "streamers". Streamer length and orientation are determined by the shape of the body of water and the prevailing wind direction and speed. In extreme cases, the heaviest bands of snowfall could be as much as 40 km wide and extend well over 100 km inland from the lake.
Within the streamer, snowfall rates can exceed 15 cm per hour. If the wind remains steady, a band of snow can hover over one location for several hours resulting in a metre or more of snow. Lake effect snowfall is very localized and 20 km on either side of the streamer, the skies might be sunny with no snow at all.
The operational consequences at an aerodrome that is experiencing lake effect snow can be significant. Delays and de-icing are certainties and diversions are likely as normal airfield operations can be severely impaired. Operators should anticipate and plan for the following:
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