So what was the big green blob over the Mississippi River this morning? We can speculate on one of three things. First, Doppler Radar can pick up biological things in the air like bugs and birds. In this case, the radar was indicating this rain was "big drops" which would go well with the idea that birds were taking flight.
Another theory is atmospheric ducting. Ducting is where the radar beam is bent as it moves away from the radar site. In this instance, the radar would be gaining elevation as it heads away from the radar, located at Davenport's Municipal Airport. As the radar beam heads off onto the horizon, the beam is "bent" back down toward the Earth. Then, the return waves are bent back to the radar. The computers plot the returns green, as if rain was falling.
It's been an hour and the radar is still showing some returns over the Louisa Co. area leading me to think that these are millions of birds in the air (probably just above the level of elevation where you'd notice them). Radar signatures like these are important to take note of, especially for pilots.
-Meteorologist Eric Sorensen
