For three days this week it snowed all around us, though we've only received 3-4" here. Snowfall in the snowbelt is obviously dependent on the vagaries of the streamers off Georgian Bay and Lake Huron. And this week, a major streamer has dumped huge amounts from Lake Huron toward London, while another streamer has snowed heavily from Collingwood southeast. Lucan, just north of London, has received nearly 5 feet of snow!! But we're in between, and it seems like just normal winter weather here.
This snow is 'lake-effect snow', caused by very cold arctic air masses moving over the relatively warmer waters of the Great Lakes. If the temperature difference between the higher, colder air mass and the air at the surface of the warmer water is big enough, atmospheric instability occurs. That is, the huge difference causes warm air (and water vapour) from the surface, to move up into the colder air, since nature doesn't like such imbalances, and always tries to even them out. (Warm air always rises, cooling as it does so, and cooler air always falls).
As the warmer air moves upward, it cools, starting the formation of snow crystals. When this air mass moves over land, it all cools, since there is no longer a source of relative warmth at the surface. The air also tends to 'pile up', as the friction of air moving over land compared to water slows down the air movement. Air has nowhere to go but up - cooling again, and adding more snow. The bigger the original temperature difference, and the longer the stretch of warm open water for the wind to blow over, the more snow that develops. Meteorologists look for a difference of at least 13 degrees celsius between the warmer and colder air, and a fetch of 100 or more km. over open water.
Such arctic air masses always tend to blow toward the south-east over southern Ontario, and then dump their heavy load of snow in a long band or streamer over land exactly as it did over Lake Huron toward London and Georgian Bay from Collingwood south-east this week. If you're in the path of a streamer, you may get excessively heavy snowfall, while areas just a few miles out of the path only get normal snowfall. Such snowfall is also more likely early in the winter, in November-December, because the temperature of the Great Lakes water hasn't cooled down much yet, so the atmospheric temperature difference is still high.
Southern Ontario, southeast of Lake Huron and Georgian Bay, (along with Buffalo and parts of New York State, east of Lake Erie) are the classic examples of 'snowbelts' in the world. Lake effect snow tends to be light and fluffy, so it easily blows around once it's on the ground, giving us white-outs and snowsqualls too.
Certainly everything here is white now, covered in a foot or more of powder snow, and with the temperature staying near -10, the snow stays light and fluffy. And it's piling up on the leeward side of our house, waiting for me to shovel it. Next time we may be in the path of the streamer, but today is bright and sunny, with a dazzling blue sky, and for now I don't have to shovel!