What causes lake effect snow?


Chilly arctic winds continue to blow over the Great Lakes today, churning up more bands of lake effect snow across areas that were already significant accumulations from yesterday — east of Lake Huron, including London and parts of the Greater Toronto Area, as well as Buffalo and Rochester.

Lake effect snow is due to a specific combination of temperatures and wind. The simple explanation is that cold, dry winds blow across relatively warm water, pick up heat and moisture from the water's surface, rise and eventually produce clouds and snow.

Of course, as with all things in our chaotic atmosphere, the full explanation is anything but simple.

[ Related: Temperatures plummet as Arctic air mass hits Canada ]

First of all, the water surface has to be liquid. If the surface has frozen over, it cuts off the source of heat and moisture for the wind. Second, the difference in temperature between the wind and the water has to be enough that the wind will pick up enough heat and moisture to produce the clouds and snow. Third, the 'fetch' of the wind has to be long enough. Fetch is the term meteorologists use to describe the distance the wind travels over the water (the wind is 'fetching' the heat moisture). If the fetch is too short, the wind doesn't have enough time to pick up the heat and moisture needed to produce snow.

Fourth, the winds need to be consistent from the same direction. If there is any 'wind shear' — a difference in wind direction between ground level and higher up — this can disrupt the entire process. Lastly, the wind speed needs to be 'just right'. If there is very little wind, the right conditions can't form to produce the bands of snow coming on shore. If the wind is too strong, it isn't in contact with the water for long enough to pick up enough heat and moisture before it makes landfall.

If the conditions are just right, though — unfrozen lake surface, at just over freezing temperatures, with a cold wind between -5°C to -25°C blowing consistently across the surface at just the right speed and from just the right direction to give a long fetch — anyone who has lived along the shores of the Great Lakes is likely quite familiar with the effect.

The areas most often affected by lake effect snows are on the southern and eastern shores of lakes, in the so-called 'snow belts'. Anywhere from Marathon, ON to Ashland, WI on Lake Superior, along the eastern shores of Lake Michigan, from Parry Sound to Godrich on Lake Huron, the Niagara Peninsula and the western Greater Toronto Area, and anywhere from Watertown to Rochester to Buffalo in New York State, as well as around the south of Lake Erie, from Buffalo to Cleveland.

[ Related: Bitterly cold winter weather chills Southern Manitoba ]

Also, it is possible for colder, stronger winds to still pick up enough moisture to produce snow and deposit it far inland, as anyone living in or around North Bay, London, Kitchener, Barrie or Peterborough in Ontario, and in or around Syracuse in New York State can attest to.

This isn't limited to around the Great Lakes, though. As long as the conditions are just right, they can happen anywhere with cold wind passing over warm water. Areas around other large lakes — Lake Winnipeg, Lake Athabasca and Lake Nipigon, for example — can can see lake effect snow, and it isn't unheard of for coastal regions to get 'ocean effect snow' as cold winds blow on shore from over unfrozen water, and regions downwind of east-coast inlets, such as Massachusetts Bay, Chesapeake Bay and Delaware Bay can even see 'bay effect snow'.