WRAL WeatherCenter Blog

At about 3:42 am there was a big circle of showers this morning and it was almost like a perfect circle. I was wondering why that was made and if it means anything.

Posted May 26, 2007 11:36 a.m. EDT

MIKE MOSS SAYS:     Tiffany,     what you saw on the radar was most likely something we call an "anomalous propagation" pattern in which the temperature and humidity structure of the lower atmosphere is such that it bends the radar beam down toward the ground, and at some distance from the transmitter the beam strikes the ground and produces a patchy area of strong echoes, even though there may be no showers in the area. This is usually most noticeable on nights when skies are mostly clear and winds are light, so that a temperature inversion forms in the lower atmosphere, and the effect is enhanced if the air near the ground is also rather humid, with much drier air just above the surface.

Sometimes the effect is sort of splotchy, other times it forms a "ring" around the radar that tends to contract through the night and then rapidly exapand in the morning before disappearing, and some others it may look like the inside of the ring was cut out with a cookie cutter. This last effect is usually the result of using a filter in the radar display software that suppresses any echo that is determined to be a ground return within a specified radius. That can lead to a sharply defined circle with no echoes inside, surrounded by echoes of varying intensity outside that specified distance.