Lake-effect snow

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 by the lake water, picks up water vapor from the lake and rises through colder air. The vapor then freezes and is deposited on the leeward (downwind) shores.

The same effect also occurs over bodies of saline water, when it is termed ocean-effect or bay-effect snow. The effect is enhanced when the moving air mass is uplifted by the orographic influence of higher elevations on the downwind shores. This uplifting can produce narrow but very intense bands of precipitation, which deposit at a rate of many inches of snow each hour, often resulting in a large amount of total snowfall.

The areas affected by lake-effect and parallel "ocean-effect" phenomena are called snowbelts. These include areas east of the Great Lakes in North America, the west coasts of northern Japan, Lake Baikal in Russia, and areas near the Great Salt Lake, Black Sea, Caspian Sea, Baltic Sea, Adriatic Sea, the North Sea and more.

Lake-effect blizzards are the blizzard-like conditions resulting from lake-effect snow. Under certain conditions, strong winds can accompany lake-effect snows creating blizzard-like conditions; however, the duration of the event is often slightly less than that required for a blizzard warning in both the U.S. and Canada.

If the air temperature is low enough to keep the precipitation frozen, it falls as lake-effect snow. If not, then it falls as lake-effect rain. For lake-effect rain or snow to form, the air moving across the lake must be significantly cooler than the surface air (which is likely to be near the temperature of the water surface). Specifically, the air temperature at an altitude where the air pressure is 850 millibars (85 kPa) (roughly 1.5 kilometers or 5,000 feet vertically) should be 13 °C (23 °F) lower than the temperature of the air at the surface. Lake-effect occurring when the air at 850 millibars (85 kPa) is much colder than the water surface can produce thundersnow, snow showers accompanied by lightning and thunder (caused by larger amounts of energy available from the increased instability), and, on very rare occasions, tornados.

Some key elements are required to form lake-effect precipitation and which determine its characteristics: instability, fetch, wind shear, upstream moisture, upwind lakes, synoptic (large)-scale forcing, orography/topography, and snow or ice cover.

A temperature difference of approximately 13 °C (23 °F) between the lake temperature and the height in the atmosphere (about 1,500 m or 5,000 ft at which barometric pressure measures 850 mbar or 85 kPa) provides for absolute instability and allows vigorous heat and moisture transportation vertically. Atmospheric lapse rate and convective depth are directly affected by both the mesoscale lake environment and the synoptic environment; a deeper convective depth with increasingly steep lapse rates and a suitable moisture level allow for thicker, taller lake-effect precipitation clouds and naturally a much greater precipitation rate.

The distance that an air mass travels over a body of water is called fetch. Because most lakes are irregular in shape, different angular degrees of travel yield different distances; typically, a fetch of at least 100 km (60 mi) is required to produce lake-effect precipitation. Generally, the larger the fetch, the more precipitation produced. Larger fetches provide the boundary layer with more time to become saturated with water vapor and for heat energy to move from the water to the air. As the air mass reaches the other side of the lake, the engine of rising and cooling water vapor pans itself out in the form of condensation and falls as snow, usually within 40 km (25 mi) of the lake, but sometimes up to about 150 km (100 mi).