How it works
Cold, dry air is stable and holds little moisture. Send it across a body of water that is warmer than the air, and the water heats and moistens the lowest layer. That warm, wet layer rises through the cold air above it, forms clouds, and snows, especially where the air is pushed up by land. The longer the run over water, the more snow. Meteorologists call it lake-effect snow in North America and sea-effect snow in Japan; the mechanism is the same.
Japan
In winter, cold air from Siberia pours south-east across the Sea of Japan, which stays comparatively warm. The air arrives on the west coast of Japan loaded with moisture and hits mountains almost immediately. The result is the heaviest, most reliable snowfall of any ski country: in our record the resorts of Hokkaido and Nagano average around 20 snowy days a month in December and January, and Hakuba’s record shows 23 in January. The cities of Aomori, Sapporo and Niigata are among the snowiest in the world for the same reason. Because the air is so cold, the snow is light and dry despite the amounts.
The effect fades fast inland. Tokyo, on the Pacific side of the same mountains, gets a few snow days a year.
The Great Lakes
Buffalo, at the eastern end of Lake Erie, and Syracuse, south-east of Lake Ontario, sit in the classic lake-effect belts, and so do the ski areas of western New York, Michigan’s Upper Peninsula and the shores of Lake Superior. The bands of snow are narrow, so one town can get 50 cm while another 30 km away stays dry. Once the lakes freeze, usually late in the winter, the effect switches off.
Why it matters for a forecast
Sea- and lake-effect snow falls in narrow bands that global weather models, with their 13 to 25 km grids, struggle to place. A forecast of “snow” for the Sea of Japan coast is reliable; the exact amount at one resort is less so, and it is often more than the model says. The daily counts in each place’s snow record are the better guide to what a typical winter brings.