Lake-effect snow is a familiar winter wonder in many "snowbelt" regions around the world. Some of the most famous snowbelts are located around North America's Great Lakes, which often pump out torrents of lake-effect snow (like in the satellite image above).
But what is this effect, exactly? How do lakes make snow, and why do some lakes make more than others?
Lake-effect snow occurs when cold air moves across a large area of warmer water — such as a frigid Canadian air mass flowing over the Great Lakes. As the cold air passes above the unfrozen and relatively warm lake waters, it draws their warmth and moisture up into the lowest level of the atmosphere. This can generate columns of warm air known as thermals, which then collide with the layer of colder air overhead, as the U.S. National Oceanic and Atmospheric Administration (NOAA) explains:
"When the rising, warmer air hits the colder air above, it condenses into cumulus clouds, then cools and sinks on either side, creating parallel cylinders of rotating air that line up in the direction of the prevailing winds over the lakes. At times when there is a large temperature contrast between the surface air and lake water, these cloud formations can deliver heavy lake effect snows on the downwind shores of the lakes."
The temperature difference between air and water is key to lake-effect snow, with a greater differential allowing the air to take in more moisture. That's why the phenomenon tends to fade in late winter, when lakes are colder and may be frozen over.
Lake-effect snow typically takes the form of "discrete, narrow bands," points out the National Weather Service (NWS), adding that these bands "are often characterized by intense snowfall and limited visibility." In the satellite image above, parallel "cloud streets" carry heavy snow out of the Great Lakes on Christmas Day 2017. Captured by NASA's and NOAA's Suomi NPP satellite, this image shows a record-breaking snow event for Erie, Pennsylvania, where more than 60 inches of snow fell in just two days.
Here's a time-lapse video showing what this snowy barrage looked like from the ground:
Because of the narrow bands that often carry lake-effect snow onto shore, the phenomenon is known for extreme variability in space and time. It's not uncommon for sunny skies "to be quickly replaced by blinding, wind-driven snowfall in a matter of minutes," according to the NWS, or for one place to receive heavy snow while only a light dusting falls a few miles away.
Related on MNN: How does snow form?