B. Geerts |
12/'98 |
The southern and eastern shores of the Great Lakes of North America are notorious for the heavy snowfall they receive each winter (Fig 1), especially from late November to early January. This is due to what is known as the lake-effect snow, and it may lead to large regional differences. For instance, 50 cm of snow may accumulate over the course of a few days near the shore, and 50 km from the lake shore the ground may be bare. Lake-effect snow occurs elsewhere as well, e.g. near Lake Baikal in Russia, but nowhere is it so pronounced and has it such an effect on ground and air transportation. The local maxima in snowfall are not due to the proximity of mountains or an ocean. The difference is not because the southern and eastern shores are cooler than the surroundings, in fact they are slightly warmer than the other shores. Snowfall typically occurs in this area after the passage of a cold front, when synoptic factors are not conducive to precipitation. A schematic cartoon of the mechanisms involved in lake-effect snow is shown in Fig 2. Fig 2 (below). Schematic diagram of how lake-effect snowfall is generated. Note that the temperatures shown are arbitrary. Lake-effect snow actually is more effective if both land and water temperatures are higher. |
Fig 1. Annual snowfall in the Great Lakes region, in cumulative inches of fresh snow. 100'' = 2.5 m |
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In more detail, these are the mechanisms, ranked in usual order of importance
Case study: 22 December, 1998
Examine the following images, all at 10 UTC (about 4 am local time)
Reference
Lake Effect Snow. The Weather Resource (web site)