The above radar picture shows the line of storms that came through BHM late Monday evening. Notice the darker blue in the ground clutter (mainly insects), indicating where cold outflow from the storms, or likely atmospheric waves, are lifting more bugs into the radar beam. Further evidence for waves is shown by the clear areas right behind the leading bands...indicating downward motion (the back side of the wave) pushing the insects back down near the ground, out of the radar beam.
Dr. Tim Coleman
UAH Research Meteorologist
Fox 6 Severe Weather Expert and Blogger
Follow me on twitter @timbhm