When towns along the Great Lakes get buried in drifts of blowing snow, as several have over the past few days, weather experts start talking about the “lake effect.”Related video above: Heavy lake-effect snow made post-holiday travel nearly “impossible” in parts of the Great LakesLake-effect snow often occurs in relatively narrow bands that dump copious amounts of snow. The weather phenomenon can drastically increase snowfall totals, and it may slam one area and leave another just miles away untouched. Over the weekend, parts of upstate New York, Pennsylvania, Ohio and Michigan saw nearly 4 feet of lake-effect snow.Here’s a look at how it works: In the United States, the lake effect typically begins when cold air — often from Canada — blows in over the Great Lakes’ warmer waters.Warming air from the lakes then pushes the moisture in the sky higher into a zone most conducive to snowfall because of …