Friday, September 01, 2006


Yeow! That's what I thought when I opened my e-mail Thursday and found this from from Phil Lewis in Alpharetta. I guess he was awakened as were most of us for the second consecutive night for some loud and in some cases, dangerous thunderbumpers. By Thursday morning, we were getting reports of several homes set on fire by lightning strikes and one tree in Midtown Atlanta that was split in two by a nighttime bolt. It was pretty unusual to have one night that turned to day, but even stranger was to have back-to-back nights. I don't know about you, but I know what I do - I bolt out of bed and stare out the window. Phil took it a step further, grabbed his camera and snagged the nifty picture.

Usually in the summertime, we wait for the sun to heat the humid air over us and by the afternoon rush hour, towering clouds form producing thunder and vivid lightning. The heavy rain that follows has a tendency to kill the updrafts that feed the storm and things quiet down. You've probably heard us say on TV that "now that the sun has set, the storm threat is dying". So what happened Tuesday and Wednesday night? To sort of quote a line from King Kong, "It wasn't beauty that killed the beast, but rather heat-energy that fueled the storm." (See, I told you I was kind of quoting the classic movie).

To get thunderstorms, you need to have two things: moisture and a way to lift the moisture high enough in the atmosphere to cool it and make clouds which eventually make rain. That's kind of a basic definition. On a warm summer day, we usually have the first ingredient in abundance (that's why they call it Hot-Atlanta). The summer sun is the force that lifts the moisture and off we go. However, at night, it's a different story. We still had the moisture and thanks to the combination of a stationary front over the state and energy disturbances riding along the front, we also had the energy to lift the moisture. The stationary front snaked from north of Gainesville through Atlanta and into Alabama. The front marks a convergence zones where opposing winds collide. Basically, its a place were moist air meets. Then, along the front, a short wave of energy took the converging air and lifted it up. That short wave started near southwest Atlanta, where the storms began and moved them up across the city and into Cobb, North Fulton and eventually Gwinnett counties. Here's what the weather map looked like Wednesday night:

The line with the blue triangles and the red semi-circles is the stationary front. The L is the low pressure center, or energy surge that rode along the front.

A truly rare event in that it happened at night. The night before, the front was a cool front and it was moving into the state. Storms formed in advance of the front Tuesday night as well with reports Wednesday morning of homes struck by lightning.

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