What phenomenon causes heavy snowfall downwind of a large lake?

Lake-effect snow is heavy snowfall produced when cold air moves across a relatively warm, unfrozen lake.

The lake transfers heat and moisture into the colder air above it. As that air becomes more unstable, convection develops and snow showers or narrow snow bands form. Winds then carry the bands toward the downwind shore, where snowfall can become extremely intense.

The Great Lakes of North America are famous for this effect, especially during late autumn and early winter. Snowfall can vary sharply over short distances: one community may receive heavy accumulation while another nearby area receives little. The greatest events usually require a large temperature difference between the lake surface and the air several kilometres above it, along with a long fetch across open water.

Lake-effect snow weakens when a lake freezes extensively, when the air warms, or when winds shift away from a favourable direction. Similar processes occur beside other large lakes worldwide.