From the ABC 7 Weather team

River fog boils out of the Potomac, a sign of fall (PHOTO)

October 10, 2011 - 05:00 AM
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Stay up late enough, and you might notice an unearthly fog developing over the Potomac River at this time of year.

Zounds! A spectral mist arises over Georgetown. (Photo: TBD Staff)

So this is why they call it Foggy Bottom! Here is the view from K Street NW at Whitehurst Freeway late last Thursday night, when temperatures shot down precipitously and condensation made every surface clammy. A white wall of shuddersome fog higher than Key Bridge arose from the Potomac, staying neatly between its shores and stretching all the way from Southwest to Georgetown and northward. That's Rossyln in the background.

Was D.C. built on an Indian burial ground? Or was it the Mist? Both are intriguing possibilities, but ABC7 meteorologist Steve Rudin has a better explanation. Here's what Rudin has to say:

Ahhh, the wonders of cool nights and a quick warm-up thanks to the sun. Basically, this is evaporation. More moisture in the air over the river (or lake, for that matter) is contrasting with the "less" moist air over land. Poof: You get river fog.

Fall is a great time for river fog; the sudden temperature dips above moisture-breathing water call these spectral mists into existence late at night. In some parts of the U.S., fog can fill riverways and valleys throughout vast territory like blood coursing through twisted capillaries. For instance, look at this satellite image of Wisconsin during a fog breakout late in August 2009. The mists paint the location of the tributaries of the Wisconsin and Mississippi rivers:

why river fog

And here's one more shot of the Potomac from last week. It's blurry, but hey, so was the fog:

river fog dc maryland virginia weather forecast monday october 2011
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