Buffalo just experienced a truly historic snowstorm.
After a huge storm on Monday and Tuesday was followed by another heavy round of snow Thursday evening, some parts of the metro region ended up with as much as seven feet in total.
This footage, shot with an aerial drone, gives you an idea the crazy amount of snow that's fallen:
But even though this is a particularly huge storm, locals are fairly used to sudden, intense bouts of snowfall. That's because the city is directly east of Lake Erie, putting it directly in the path of a meteorological phenomenon called lake effect snow.
How lake effect snow works
During the early part of winter, the Great Lakes are often warmer than the surrounding air, because water holds residual autumn warmth more effectively than air. The difference is generally greatest during cold snaps in the early winter.
When a particularly cold air mass travels over a lake, the water slightly warms up the bottom layer of it. Because warm air can hold more moisture than cool air, the moving air mass then begins evaporating water off the lake, forming clouds. Eventually, these clouds rise to the top of the air mass, because warmer air is less dense.
Then, as the air finishes crossing the lake and passes back over land, it drops slightly in temperature again, because the land is cooler. In some places, it might also get pushed upward in elevation a bit by a higher landmass. Both lower temperature and increased altitude mean that the clouds are less capable of holding in moisture — so they often release tons of it, in the form of fresh snowfall.
For a particularly vivid example of this, here's a time lapse of what it looked like in downtown Buffalo on Tuesday, as ominous snow-bearing clouds rolled in off the lake:
A few factors have to be in place for all this to happen. There needs to be a pretty big temperature difference between the lake and the air (usually around 20-30°F), and the air needs to get blown across the water for a sufficient distance to pick up lots of moisture.
Because winds typically move from northwest to southeast in this area, the places that get hit with this snow (called snowbelts) are all on the southeastern sides of the lakes: Western Michigan, the Upper Peninsula, and Western and Upstate New York.
But even within a particular event, the characteristics of wind direction, speed, and air temperature can lead to extremely localized bands of snow. In Buffalo, for instance, the southern part of the city and suburbs got many feet of snow over the past few days, but northern suburbs just got a few inches. You can see that difference especially clearly in the radar:
This radar loop in Buffalo is bleeping insane. Meteorology textbooks should save this lake effect loop. WOW. http://t.co/r1ZYbd7gAR— PhillyWx.com (@phillywx) November 18, 2014
Lake effect snow also happens in Canada, around the Great Salt Lake, and in several other places around the world — essentially, anywhere you have colder air masses getting blown over a warmer body of water.
Residents of Buffalo will eventually get a much-needed reprieve once Lake Erie freezes over sometime this winter — once that happens, the lake effect is much less likely to happen, because the moving air masses can't evaporate water off of ice.
Correction: This article previously stated that the lake effect was impossible over a frozen lake. It's possible, just less likely to occur.