Lake-effect rain

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Lake-effect rain happens when cold air moves over a lake that’s still warm. As the air travels over the water, it heats up moisture from the lake, rises, and cools enough to condense into rain. The result is narrow bands of rain that form downwind of the lake. If the air near the ground stays warm enough (above freezing), the precipitation falls as rain rather than snow.

key points:
- For lake-effect rain to form, the air over the lake must be significantly cooler than the air above it and much cooler than the air near the water’s surface.
- These rain bands can bring heavy rain quickly and, in extreme cases, flash floods, thunder, lightning, or even waterspouts.

Although it’s famous around the Great Lakes, lake-effect rain can occur anywhere with a large, warm lake that remains warmer than the air in autumn and early winter.

Sea-effect rain works the same way over the ocean. Cold air moving over warm sea water rises and cools, forming clouds and rain near the coast. The strength of sea-effect rain depends on the vertical temperature gradient between the surface and higher in the atmosphere. A steeper gradient means more intense rain; a gentler gradient means lighter rain. As the bands move inland, they usually weaken.

In general, the amount and severity of lake- or sea-effect rain are driven by how much the air cools with height and how much moisture the water surface provides.