Download Lake-effect Snow Introduction Lake-effect snow is produced in the

Lake-effect Snow
Introduction
Lake-effect snow is produced in the winter when cold winds move across long expanses of warmer
lake water, providing energy and picking up water vapor which freezes and is deposited on the lee
shores. The same effect over bodies of salt water is called ocean effect snow, sea effect snow, or even
bay effect snow. The effect is enhanced when the moving air mass is uplifted by the orographic effect
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 and often bringing
copious snowfall totals. The areas affected by lake-effect snow are called snowbelts. This effect occurs
in many locations throughout the world, but is best known in the populated areas of the Great Lakes
of North America.
If the air temperature is not low enough to keep the precipitation frozen, it falls as lake-effect rain. In
order 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 the altitude where the air pressure is 850 millibars (roughly 1.5 vertical kilometres)
should be 13°C lower than the temperature of the air at the surface. Lake-effect occurring when the
air at 850 millibars is much colder than the water surface can produce thundersnow, snow showers
accompanied by lightning and thunder (due to the larger amount of energy available from the
increased instability).
Formation
Lake-effect snow produced as cold winds blow clouds over warm waters.
There are several key ingredients that 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.
Instability
A temperature difference of 13°C between the lake temperature and 850 millibar level, or 1,500
metres (4,900 ft) above sea level, 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 will allow for thicker, taller lake effect
precipitation clouds and naturally a much greater precipitation rate.
Snow and Ice Cover
As a lake gradually freezes over its ability to produce lake effect precipitation decreases for two
reasons. Firstly, the open ice free liquid surface area of the lake shrinks reducing fetch distances while
secondly, the water temperature nears freezing reducing the overall latent heat energy available to
produce squalls. In order to end the production of lake effect precipitation, a complete freeze is often
not necessary.
Even when precipitation is not produced, cold air passing over warmer water may produce cloud
cover. Fast moving mid latitude cyclones, known as Alberta clippers often cross the Great Lakes. After
the passage of a cold front, winds tend to switch to northwest, and a frequent pattern is for a long
lasting low to form over the Canadian Maritimes which may pull cold northwestern air across the
Great Lakes for a week or more. Since the prevailing winter winds tend to be colder than the water for
much of the winter, the southeastern shores of the lakes are almost constantly overcast, leading to
the use of the term The Great Gray Funk as a synonym for winter. These areas allegedly contain
populations that suffer from high rates of seasonal affective disorder, a type of psychological
depression thought to be caused by lack of light.
Phenomenon in the Great Lakes Region
Cold winds in the winter typically prevail from the northwest in the Great Lakes region, producing the
most dramatic lake-effect snowfalls on the southern and eastern shores of the Great Lakes. This lakeeffect produces a significant difference between the snowfall on the southern/eastern shores and the
northern/western shores of the Great Lakes.
Lake-effect snows on the Tug Hill Plateau (east of Lake Ontario) can frequently set the daily records for
snowfall in the United States. Syracuse, New York is directly south of the Tug Hill Plateau and receives
significant lake-effect snow from Lake Ontario, averaging 115.6 inches (294 cm) of snow a year, which
is enough snowfall to often be considered one of the "snowiest" large cities in America. The
communities of Redfield in Oswego County and Montague and North Osceola in Lewis County, all on
the Tug Hill Plateau, average over 300 inches (760 cm) of snow a winter. In February, 2007, a
prolonged lake-effect snow event left 141 inches (360 cm) of snow on the Tug Hill Plateau.
A small amount of lake-effect snow from the Finger Lakes falls in upstate New York as well, until those
lakes freeze over. The Appalachian Mountains and Atlantic Ocean largely shield New York City and
Philadelphia from picking up any lake-effect snow; snow there tends to come from storm systems
mixing with cold weather.
Lake Erie produces a similar effect for a zone stretching from the eastern suburbs of Cleveland
through Erie to Buffalo. Remnants of lake-effect snows from Lake Erie have been observed to reach as
far as Garrett County in western Maryland.