Simultaneous experiments have been conducted with ultrasensitive radars at 3.2‐, 10.7‐, and 71.5‐cm wavelengths to observe ‘angel’ echoes from apparently clear air and to diagnose the scattering mechanism. Two types of echo layers have been found. Type I echo layers appear incoherent at long ranges or with wide beams but are seen to be composed of discrete coherent echoes when viewed with high resolution. The cross sections of the discrete targets vary roughly between the inverse first and second power of wavelength between 3 and 10 cm and between the inverse third and fourth power between 10 and 71 cm. Both the magnitude of the cross sections and the wavelength dependence are consistent with the hypothesis that the targets are large insects. Type II echo layers are composed of incoherent echoes at all ranges, show little or no wavelength dependence, and are generally undetectable at 3 cm. This type of layer is attributed to refractive index fluctuations which are maximized near stable layers—layers of maximum gradient in refractivity. Their reflectivities are consistent with mean‐square fluctuations in refractivity measured directly in similar layers and with that deduced from forward‐scatter measurements.

There has long been disagreement over the nature of many of the socalled "angel" echoes which are commonly observed by radar from regions of apparently clear atmosphere (1). Birds, insects, and atmospheric refractivity perturbations are usually mentioned as sources of these echoes; however, the relative importance of one source in a given series of measurements is clouded by the almost total lack of quantitative measurements of radar backscatter for either known insect flights or refractivity perturbations. The object of the experiments discussed here was to extend our basic knowledge of the radar backscattering properties of insects in free flight and knowledge of the characteristics of these flights, in order to distinguish echoes from insects from those due to clear air phenomena.

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