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Oasis
Thu Jul 29, 2004
Listen in RealAudio 
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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