Mount Cheaha, the highest point in Alabama at 2407 feet above sea level, was visible on the WSR-88D radar located at the Shelby County Airport this morning. I grabbed this screen shot of the radar around 7:45 am and added an anotation to show the "echo" which is Mount Cheaha.
How does this happen? The radar beam is assumed to travel in a straight line, but in reality, atmospheric temperature and humidity profiles affect the radar beam. This means that the beam does not travel in a straight line, but bends. The bending can be upward (subrefraction) or downward (superrefraction). Today, the lowest beam in the radar volume scan is being bend downward just enough to strike Mount Cheaha and return some energy thus painting at "echo." Of course we know that echo is not rain but a geographic feature.
This situation illustrates a good point. If you were to look at radar alone, you might conclude that a small shower was occuring in the vicinity of Mount Cheaha. However, when you combine radar and satellite imagery or the sounding from the Birmingham radiosonde run, you would see that the correct conclusion is beam bending and no shower exists.
Special thanks to the NWS forecasters at Birmingham for noting this occurrence in their forecast discussion.
-Brian-
on May 6, 2006, 10:02 am
Quick question:
Does that mean the "echoes" shown up in Calhoun County correspond to geographic features as well? They seem roughly to correspond with Coldwater Mountain, Blue Mountain and some of our other local peaks.
Thanks!
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