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WxBlog / 20100110A

Uncommon situation with satellite IR imagery this morning

Due to the thick snow cover, and an arctic-like high over the region, the temperatures over the Great Lakes regions hovered near or below zero (F) overnight. However, most of the Great Lakes remain unfrozen, which means the water temperatures are at least 30 degrees warmer than the surrounding landmass.

At the same time, light NNW lower level winds were blowing across Lake Michigan into Indiana. The warmer moist air flowing off the lake overflowing the much colder snow-covered ground formed low clouds (and probably fog near the lake).

In this strange quirk of winter and geography, the clouds were warmer than the surrounding air and showed up on the infrared satellite imagery darker than the surrounding clear areas. We are use to detecting clouds because they are brighter than the ground; as in the accompanying image the higher level clouds across WI and northern MI are brighter than the surrounding (though still frigid) surface areas.

Shortly after midnight here in Fort Wayne, the temperature was 4F. Then these clouds moved over and within an hour we had climbed to nearly 14F.

0945Z IR Image

You can tell the brighter area (specifically Illinois) was clear, because you can distinctly make out stationary land features like rivers and lakes. Also, if you view a loop (time-lapse) of this morning's IR satellite images, the darkness is clearly a moving, slightly morphing, blob.

The same effect was occurring over eastern Ohio with winds blowing across Lake Erie; but the clouds did not extend as far because the wind was blowing across the narrow axis of Lake Erie and thus did not have a chance to warm up as much as the winds that were blowing across nearly the entire north-south length of Lake Michigan.

Update 2010-02-02

Actually, it apparently isn't that uncommon. Nearly every day since then I've noticed low clouds warmer than the surrounding clear (and usually snow covered) areas. I still find it "fascinating".

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awc 2010-01-10 12:35
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