What is lake-effect snow, and why can it be so extreme?
When cold air sweeps over warmer lake water, it can dump staggering amounts of snow on narrow bands downwind.
Few weather phenomena can bury a neighbourhood under several feet of snow while leaving a town a short distance away barely dusted. That is the signature of lake-effect snow, a localised but extraordinarily intense form of snowfall that can paralyse communities downwind of the Great Lakes and other large bodies of water.
Lake-effect snow forms when a mass of cold, dry air sweeps across a relatively warm, unfrozen lake. As the cold air moves over the warmer water, it picks up heat and moisture from the lake surface. This warmed, moistened air rises, cools and condenses into clouds, dumping its moisture as snow when it reaches the colder land on the far shore.
The result can be astonishing. Narrow, intense bands of snow can set up for hours or days, producing snowfall rates of several inches per hour and total accumulations measured in feet, while areas just outside the band see little or nothing. Cities like Buffalo are famously prone to such events.
Several factors control the intensity, including the temperature difference between the air and water, the distance the air travels over the lake, and the wind direction, which determines where the bands set up.
Lake-effect snow typically fades once the lakes freeze over, cutting off the supply of moisture. Interestingly, as warming delays that freeze-up, some research suggests the season for lake-effect snow could lengthen, a reminder that a warming climate can produce complex and sometimes counterintuitive effects on winter weather.