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
In more detail, these are the mechanisms, ranked in usual order of importance
- Heating. The water of the Great Lakes lags behind the atmosphere in cooling through the fall and early winter. The heating from below results in static instability, especially during cold outbreaks. This instability mixes near-surface warm, moist air into the lowest 1 to 1.5 km, sometimes more. Rising air quickly reaches saturation, and the result is shallow cumuliform clouds, often aligned in bands parallel to the low-level wind. By January, ice covers most lakes, at least in part, cutting off or reducing the heat supply. Lake Erie often freezes entirely because it is more shallow.
- Moisture. The lake surface evaporates, which is very effective when the wind is strong and the air dry (Dalton's equation, Note 4.E). The cold air from Canada has a very low vapour pressure. Also, also strong winds cause spray, facilitating evaporation.
- Wind Fetch. The length of trajectory of the wind across the lakes has a great bearing on the development of lake-effect snow. The greater distance the wind blows over the warm water, the greater the convection. Three of the five lakes, those with the most population centers, are relatively long and narrow. Winds blowing along the length of these lakes have a long trajectory over water, whereas a 30 degree windshift will bring the winds across the lake, not only shortening the trajectory considerably, but also moving the lake-effect snow to a different site
- Frictional Difference. The stress applied to the atmosphere from the surface is much greater over a rough land surface than over a relatively smooth lake water surface. When the surface winds blows from lake to land, it encounters increased friction, slowing the surface wind over the land, resulting in surface convergence and lifting. Since stress varies with the square of the wind speed, this effect is greater with strong winds.
- Upslope lift. In some localities, wind blowing from a lake onshore is forced to climb up hills. This is not a major factor in precipitation along the immediate lakeshore, but affects some more island locations. Certainly this effect is important in the case of Lake Baikal in Siberia.
- Land breeze. Sometimes the lake-effect snow is concentrated along a narrow band due to mesoscale flows around the lake, in particular a landbreeze from one or opposing shores, e.g. when a weak northerly gradient wind blows along Lake Michigan.
- Large-scale forcing (potential vorticity advection, isentropic uplift ...). The general cyclonic nature of an airmass, which supports development of precipitation anywhere, may enhance lake-effect snow.
Case study: 22 December, 1998
Examine the following images, all at 10 UTC (about 4 am local time)
Satellite infrared imagery: note the cold cloud tops of a frontal system, the surface temperature difference across this front, and the warm surface of the Great Lakes (where they are not cloud-covered, i.e. on the western side)
- Station observations: note the strong westerly/northwesterly winds around the Lakes, and snow records east of several Lakes. Also, a cold front is obvious in the southeast corner of the image, with southwesterly winds, temperatures above 60°
C) and high dewpoints ahead of the front and northwesterly winds, temperatures near freezing, and dry air behind the front
- Synoptic analysis: note the frontal rainband, the low associated with it, the cold front, and the lake-effect snow.
- Radar observations: shown is the radar reflectivity, blue values are the lowest (up to 20 dB) and green values higher (20-30 dB). The higher the reflectivity, the heavier the precipitation (Note 15.A). Several bands can be seen. Lake-effect snow is characterised by low reflectivities, even if the snowfall is heavy, because its clouds are shallow, so ground-based radars may look above the snow-generating clouds, even when they scan near the horizon, because of the Earth's curvature. The radar does not distinguish between rain and snow.
- Satellite visible imagery: note the several cloud bands. This picture was taken after daybreak, a few hours later, when the cold front has moved further east.
Lake Effect Snow. The Weather Resource (web site)