Lake-Effect Rain

Lake-effect precipitation forms when a cold air mass moves across a long stretch of warmer lake water, which heats the lowest layer of air and lets it pick up moisture; that moist air then rises through the colder air above, cools, and falls as snow or, in warmer conditions, rain on the downwind shores of the lake. It needs a temperature difference of roughly 13 degrees Celsius between the lake surface and the air about 1,500 metres up to become unstable, and the cold air must cross at least about 100 kilometres of open water to pick up enough moisture; wind shear under about 30 degrees produces the most organized, intense bands, while shear over 60 degrees yields only weak flurries. The effect is strongest around the Great Lakes of North America but also occurs near the Black Sea, Caspian Sea, Baltic Sea and Lake Baikal, along the west coast of Japan and around the Great Salt Lake, and the same mechanism over salt water produces ocean-effect and bay-effect snow; in rare extreme cases it can even generate thundersnow or tornadoes.