From Glossary of Meteorology
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A radar echo caused by a physical phenomenon not discernible by eye at the radar site.

Angels may appear as coherent or incoherent echoes. When diffuse and incoherent appearing, they are sometimes called ghost echoes. Angel echoes observed by radars with wavelengths of about 10 cm and less are usually caused by birds or insects. Radars with longer wavelengths and radar wind profilers, which operate in the UHF and VHF radio frequency bands, regularly detect echoes from the optically clear air that are caused by spatial fluctuations of the atmospheric refractive index.
See Bragg scattering;
compare clear-air echo.