Cold air is usually dry air. That’s why really cold places don’t get a lot of snow. However, a large lake or ocean can change all that.
When cold Arctic air sweeps across open water, it picks up moisture. When that cold air sweeps across a lot of open water, like the Great Lakes, it picks up a lot of moisture. It holds onto that moisture until it hits dry land. Then all that moisture lets go and falls as snow, a lot of snow!
The Great Lakes snow machine turns on the first time that the air temperature falls below freezing. This is when we get the heaviest snowsqualls, because “warm” cold air can hold a lot more moisture than colder air. It stays on as long as the windward Great Lake remains unfrozen. After the lake freezes, the moisture source is cut off and the snow squalls stop.
Lake effect snow squalls only hit the areas which are downwind from the unfrozen lake. All other sides of the lake are completely unaffected. Areas which are to the east and south of the lake are usually the hardest hit. Inside this snowbelt, snowsqualls will hit whenever there is a steady wind off the lake, which means there will be some snow nearly every day until the lake freezes.
The Niagara Peninsula and the Bruce Peninsula get it really hard because they are nearly surrounded by major bodies of water. As long as at least one of those lakes is unfrozen, those places are going to get lake effect snow, no matter which direction the wind is coming from.
The snow comes off the lake as one or more parallel streamers which line up with the wind. Each streamer is so narrow that one side of the city can be in whiteout conditions while the other side has bright sunshine.
This can be very dangerous for drivers, who can go from clear skies to whiteout conditions in seconds. Both the 401 and 402, Canada’s two largest highways, go through the snowbelt and are prone to snowsqualls. In mid-December of 2010, both the 401 and 402 had to be shut down between London, Sarnia, and Chatham when snowplows could not keep up with the falling snow. Over three hundred cars and trucks were not going anywhere until the snow stopped. All emergency transportation on these two major highways was by snowmobile!
Places inside the streamer can get 2 cm of snow or more per hour. Streamers usually wobble back and forth so that no place gets hit for longer than a day or so. That kind of streamer usually dumps about 20 or 30 centimetres of snow before it leaves.
However, sometimes a streamer stays in one spot for a long time. That can dump unbelievable amounts of snow in that narrow area. In early December of 2010, London, Ontario, got a metre of snow in three days, but nothing outside 20 km east, west, or south of London saw a single flake of snow. Just north of London, during the same squall, Lucan got stuck under the worst of it for a day or so longer, and got nearly two metres of snow!
Lake Erie and Ontario usually freeze over first. This means that the Niagara Peninsula and upstate New York get a lot of early season snow from lake effect snow, but not so much later in the season. Upstate New York also gets some lake effect snow from Lake Champlain.
Lake Michigan freezes over next. This spares Michigan’s lower peninsula after about January.
Lake Huron is slow to freeze over, although Georgian Bay, which is almost as big as a Great Lake itself, usually freezes over completely by January. Lake Superior never completely freezes over. Toronto gets spared once Georgian Bay freezes, but everything downwind of these lakes gets lake effect snowsqualls for most of the winter!
The weather patterns in this region mean that passing storms are usually followed by north or northwest winds, perfect for generating lake effect snow. Those snowsqualls often leave much more snow than the original storm!