What Does Lake-Effect Snow Actually Mean?

We hear it a lot in Chicago. What does "lake-effect snow" mean, exactly?

When meteorologists talk about "lake-effect snow," it doesn’t necessarily mean a snowstorm. It could mean anything from flurries to several feet of snow.

The amount of snow produced in a lake-effect situation all depends on the set-up. As a rule of thumb, the temperatures about 5,000 feet above the surface need to be at least 23 degrees colder than the water temperature. 

If the air mass does not meet the 23 degree Fahrenheit threshold, the clouds will not be able to absorb enough heat and moisture from the lake to produce enough snow. Warm, moist air from the lake rises into colder, drier air above the surface. This rising motion creates the snow bands.

The amount of time the cold air mass travels over the warmer lake is also important. This is called the fetch. The longer the fetch, the thicker the clouds grow and the more snow develops downwind.

You need optimal wind direction for the most lake-effect snow to develop. The optimal wind direction for Lake Michigan is winds out of the north and north-northeast. 

Lake-effect snow bands are very thin and long. The average width of a lake-effect snow band is about 10 miles, but the length can range from 30 to 250 miles long.

This all depends on how strong the winds are to carry the snow bands inland. So, if the winds are strong then the heaviest snow falls farther inland and if the winds are weaker the heavier snow stays near the lake.

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