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A radar echo caused by a physical phenomenon not discernible to the eye.

Angels are usually coherent echoes and sometimes of great signal strength (up to 40 decibels above the noise level). They have been ascribed to insects flying through the radar beam, but have also been observed under atmospheric conditions which indicate there must be other causes. Studies indicate that a fair portion of them are caused by strong temperature or moisture gradients, or both, such as might be found near the boundaries of bubbles of especially warm or moist air (see blob). They frequently occur in shallow layers at or near temperature inversions within the lowest few thousand feet of the atmosphere.


This article is based on NASA's Dictionary of Technical Terms for Aerospace Use