Doppler radars require targets to determine the motion of the air. Outside of regions where precipitation is falling, there would be no targets if there were no insects. Airborne insects turn out to be very good tracers of air motion since, on average, they blow along with the wind. The returns from insects allow meteorologists to see air motions outside the storm circulation which in many cases is important for predicting where new storms are likely to occur. Turbulence provides another way in which electromagnetic energy from a radar can be back-scattered.
Turbulence is associated with variations in density in the atmosphere.
When variations in density occur on a scale of half the wavelength of the radar, energy is scattered through a process called diffraction . Radar echoes in a clear atmosphere will be more common on days when the lower atmosphere is unstable, as when there are thermals present, or when the wind increases rapidly with height just above the ground, so that there is mechanical turbulence. Radar returns are due to insects, turbulent motions, and ground targets. These echoes, which extend out to about 100 kilometers from the radar, allow the radar operator to see the air motion. This second picture shows the corresponding radial velocity field.