Lake-effect snow

B. Geerts


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

  1. 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.
  2. 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.
  3. 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
  4. 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.
  5. 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.
  6. 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.
  7. 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)



Lake Effect Snow. The Weather Resource (web site)