Lake-effect snow is produced during cooler atmospheric conditions when a cold air mass moves across long expanses of warmer lake water, warming the lower layer of air which picks up water vapor from the lake, rises up through the colder air above, freezes and is deposited on the leeward (downwind) shores.
Lake effect snow is caused when a very cold winter air mass flows over the relatively warmer waters of a large lake. Intense evaporation from the lake surface under these conditions forms convective clouds that cannot contain all of this water, and some of it falls back to the surface as snow.
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