Lake Superior
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During spring and summer, you expect that water will evaporate from lakes and rivers. And most lakes do lose considerable moisture, But Lake Superior is an exception. It actually gains water from the air through dewfall. Hi. I'm Bryan Yeaton and this is The Weather Notebook.

Lake Superior straddles Canada and the United States in the Midwest. It is the largest of the Great Lakes, not only in surface area but also in total water volume. .After cooling each winter, Superior's waters take a long time to reheat during spring and summer. Except in the shallower regions, most of Superior's surface water -- and the air just above it -- remains below 55 degrees F all summer long.

During summer, hot and humid air from the Gulf of Mexico frequently flows northward and crosses the Superior basin. When it does, the cold lake water cools the air on contact to well below its dew point (usually 60 degrees or more). When this happens, the water vapor condenses and forms dew upon the lake surface.

This is the same process that forms a layer of water on your iced tea glass or cold soda can on hot days. But of course, you don't see it on the lake surface like you do on the glass or when dew forms at night on the grass. Nonetheless, it is there.

This condensed water, or dewfall, on the surface of Lake Superior can add several inches of water per month. Uniformly deposited over the whole 32,000 square miles of Lake surface, the water gain amounts to over 550 billion gallons, and that is a Superior moisture gain!

Thanks today to contributing writer Keith Heidorn of Victoria, British Columbia. The Weather Notebook is a production of theMount Washington Observatory and supported by Subaru.