U.S.A. National committee report, fifteenth URSI general assembly, Munich, September 1966: Commission 2, radio propagation in nonionized media: Progress in radio propagation in nonionized media

In 1965 there was a strong reawakening of interest in “angel” or clear-air radar echoes. Atlas [1965] postulated that “dot” or point-like angles were reflections from smooth, sharp boundaries at the apex of convective thermals. Focusing from the curved surface reduced the requirement on the index gradient to 1 N unit through 1 cm; however, the requirement of smoothness to a fraction of a cm over the first Fresnel zone is still severe and lacks experimental confirmation. Dot angel activity at night requires another explanation. Hardy, Atlas, and Glover [1966] and Deam and LaGrone [1965] conclude that dot angel characteristics are explainable only if the targets are insects. These conclusions stem from considerations of absolute cross section and wavelength dependence. Others, using measured Doppler velocities of the dot angels, concur in the insect hypothesis, stating that the weak flyers tend to drift with the mean wind while the stronger flyers may actually have a mean velocity relative to the wind.