Abstract
Two radars of different characteristics were used on a clear night in early summer to investigate the nature of the clear air echoes. It is deduced that most of the echoes on this evening were due to insects rather than atmospheric turbulence or birds 1) by the magnitude of differential reflectivity; 2) by the lack of characteristic bird wing beat signature; 3) by the inability to recognize birds using a collimated spotlight; 4) by the strength of the signal return; and 5) by the migration schedules of birds in central Illinois. The CHILL radar measured differential reflectivity, and showed that, at low levels, the insects were flying with a common heading. At higher levels, the differential reflectivity showed no preferential heading of the insects, but was still greater than for atmospheric turbulence. It is proposed that differential reflectivity provides a means of differentiating between echoes due to atmospheric turbulence and insects.
