Weather Notebook
Bryan Yeaton

Oasis Effect
Thu Jul 14, 2005

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The impact of the Great Lakes on regional weather can make headlines, as when six feet of lake-effect snow buries Buffalo. But, another phenomenon in the late spring and summer has a quieter, gentle impact, which some meteorologists call the "Great Lakes Oasis Effect."

Hi, I'm Bryan Yeaton and this is The Weather Notebook.

The Oasis Effect results in a cooling of the Great Lakes coastal zones, and over their waters themselves, often reducing the oppressiveness of hot, sticky weather for many residents. Here's why.

The Lakes cool slowly in late autumn and early winter, but eventually the topmost layer water falls to around the freezing mark. As the year moves into spring and summer, the Lakes are also slower to warm, and remain cold relative to daytime air temperatures over land.

When the wind blows over the cold lake waters, its air temperature is reduced, providing natural air-conditioning with this onshore flow. In addition, the temperature contrast between land and lake may produce afternoon lake breezes -- cool winds that blow inland from the lake.

If you live along the Great Lake shores, the phrase "cooler near the lake" commonly occurs in your local weather forecasts. When onshore breezes flow, coastline temperatures often drop 10-20 degrees below sites farther inland.

The cooler lake waters also inhibit the growth of afternoon cumulus clouds over lake waters and along the windward coast. Summer satellite observations often show the Lakes and shore regions free of clouds, while the surrounding land is dotted with popcorn cumulus.

Also, the cold waters may act like brakes on thunderstorms moving out over the lakes by knocking out a prime energy source, strong daytime surface heating.

The Weather Notebook is a production of the Mount Washington Observatory. It is supported by the National Science Foundation and Subaru, Driven By What’s Inside.

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