Multiwavelength backscatter from the clear atmosphere

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.