In temperate zones whose weather is dominated by frontal cyclonic systems, frigid, dry air masses blow over water bodies of a particular size after the passage of a cold front during the winter months, setting up a chain of events that causes snow to fall on the lee shore.
Students of physics will recall that liquid water has a higher heat-carrying capacity than just about anything else. As a result, an open body of water tends to remain warmer than nearby land. This sets up an assembly line of snow production as wind blows across it:
- The cold, dry air draws up warmth and moisture.
- The added warmth causes instability in the air, and convection cells develop under a thermal inversion capping the water body.
- Bands of cumulus clouds form (across the direction of the wind) due to condensation at the top of the cells.
- As the air mass passes back over land, the air is slowed down by the land's rougher surface (frictional convergence), chilled by the colder land surface (thermal convergence) and squeezed out by higher topography (orographic convergence), causing snow to fall.
1 2 3 4
_ -
_ -
_ - ___ ______________,-
_ - __ ( ) ( ,--------'
- _ ( ) ( ) ( ) * * *
-> \| _ _ _ ( ) ( ) ( ) ( ) * * * *
/| /| (_) (__) (____) (______) (_____)* * * * *____
-> * *__.----'
----------. .-'
|wwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwww|
`.___________________________________________________.'
Sometimes, convection will instead occur around the axis of the blowing wind ("horizontal roll convection"), and snow will form in multiple bands parallel to the wind. Also, lake-effect instability in the air can often increase snowfall during the passage of the front itself, a phenomenon called "lake enhanced snow."
The Great Lakes of North America are the water bodies most commonly associated with lake-effect snow, but other bodies will cause similar snows in the right circumstances. A water body needs to be at least 20 km in order to create enough instability to cause snow to fall. New York's Finger Lakes can cause lake-effect snow if the wind is blowing in the right direction; Chesapeake Bay can cause snow on the Delmarva Peninsula, the Sea of Okhotsk can cause snow in Hokkaido, and the Great Salt Lake can cause snow in Salt Lake City.
But the big snow areas are on the eastern and southern shores of the Great Lakes. The snows in Buffalo and Cleveland are legendary, and the Keweenaw Peninsula in Michigan and the Tug Hill Plateau near Watertown, New York receive particularly heavy snowfalls.
Many other factors affect the production of lake effect snow, but we will only mention wind direction, which in the case of the Great Lakes also determines the fetch of the lake effect. If we look at Lake Erie:
N|F B = Buffalo
_.-'. NF = Niagara Falls
L _.-' | B E = Erie
/ e / C = Cleveland
__.--._/_ i / S = Sandusky
/ r .' D = Detroit
_/ E .' W = Windsor
D|W / e .' E L = London
|___/ k .'
/ a _.-'C
| L .-'
T`-._____.--'
S
- If the wind is blowing from the northwest (the front just went through, and the low is in in New England or the Maritimes), more or less "across" the lake", the lake effect meets the northwest slopes of the Allegheny Plateau, producing a snow in Cleveland and Erie. If the effect is large enough, snow will continue across Pittsburgh and the Appalachian Mountains into the Mid-Atlantic region, sometimes reaching as far as Baltimore, Washington, and Philadelphia.
- If the wind is blowing from the southwest (jet stream directed-winds after the front), the lake effect builds along the whole long axis of the lake, causing a massive snow event in Buffalo.
- If the wind is blowing from the northeast (warm front in advance of the low), Sandusky and Toledo may get snow.
Greg Byrd, LAKE-EFFECT SNOW
Cooperative Program for Operational Meteorology, Education and Training, National Center for Atmospheric Research
http://www.comet.ucar.edu/class/smfaculty/byrd/faculty98w.ppt