Lake-effect snow is produced during cooler atmospheric conditions when a cold air mass moves across long expanses of warmer lake water. The lower layer of air, heated up by the lake water, picks up water vapor from the lake and rises up through the colder air above; the vapor then freezes and is deposited on the downwind shores. But Lake-effect snow can actually occur when a lake is partially or even total frozen. That is because directional wind shear is one of the most important factors governing the development of snow squalls; environments with weak directional shear typically produce more intense squalls than those with higher shear levels. Frictional convergence is the major factor in producing the squalls. When the wind blows over an expanse of water – or even frozen lake – the air is flowing unimpeded, when it reaches the shoreline it slows as it comes into contact with the land and causes friction. There is only once place for the air to go and that is up. The lifting causes clouds and precipitation. When the wind blows in such a way that is turns counter-clockwise, the same direction as large-scale storms – that also aids in lift. When the lake is frozen the added element of temperature contrast and available moisture is added to the mix. This is most common around the Great Lakes in the United States and Canada. The snowfall totals can be prodigious, in a short period of time. On January 5, 1988 Heavy lake effect snows on the east side of Lake Ontario in upstate New York. So much snow fell that Interstate 81 was closed from Pulaski to Watertown for a week. As much as 45” of snow fell in less than 12 hours.
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