In principal clear air echos observed by the radar can be also related to turbulence in the air. However, the intensity of the echoes has been so high that it is quite right to say that these echoes are mainly caused by insects.
Weather radar measurements do not normally reach the ground level, since the antenna is pointed so that it shoots somewhat over the horizon to reduce interference by the near lying ground areas (buildings, hills etc.). Typical lowest elevations are of the order of 100 metres. This in turn means that insects that are observed usually fly outside their boundary layer i.e. drifting with the wind being unable to control the destination of their flight. Therefore practically all insects that the weather radar detects are migrating in the sense that they have for some reason risen to heights where the wind controls their travelling.
The size of single big insects detected by radar can be roughly estimated according to the echo intensity. A more sophisticated analysis of the echo could even tell the wing beat frequency, and with that more exact information about the species. In practice most of the clear air echoes observed by a weather radar come from many insects simultaneously due to the extreme sizes of radar measuring volumes. On the other hand this extends the ranges where insects can be observed, but on the other hand limits the possibilities of having even some information about what the insects are. Therefore other entomological observations are needed together with radar observations if the radar data is to be used in studying insect migrations.
A study using weather radar and insect traps has been done related to migration of birch aphids into Finland in May, 1988. Large insects can be separated even inside a radar measuring volume by Doppler spectra. A recent moth migration into Finland got much public interest, and was actually first observed by the weather radar of the Department of Meteorology.