METEROROLOGICAL “ANGEL” ECHOES

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  • 1 Geophysics Research Directorate, Air Forece Cambridge Research Center
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Abstract

New observations of a variety of “angel” echo phenomena were made during 1957. These may be grouped into four types: (1) echoes from thermal columns below cumulus clouds; (2) mantle-shaped echoes associated with the boundaries of cumuls clouds; (3) layer echoes associated with sharp vertical gradients or minima in moisture aloft; and (4) anomalous low-level echoes believed to be produced by the boundary surfaces between differentially moistened surface air over adjacent cold and warm water. These observations and those previously reported of sea breeze angles constitute impressive empirical proof of the association between angel echoes and meterologically induced gradients in refractive index. However, it is still not possible to account for the observed angel cross-sections theoretically using “reasonable” gradients of refractive index. The regular observation of elevated blobbly angel layers indicates that conditions are frequently suitable for enchanced forward-scatter beyond the horizon of otherwise line-of-sight electromagnetic waves, Furthermore, the data testify to the feasibility and remarkable foresightedness of Friends's 1939 proposal to use radio reflections for the continuous sounding of air-mass boundaries.

Abstract

New observations of a variety of “angel” echo phenomena were made during 1957. These may be grouped into four types: (1) echoes from thermal columns below cumulus clouds; (2) mantle-shaped echoes associated with the boundaries of cumuls clouds; (3) layer echoes associated with sharp vertical gradients or minima in moisture aloft; and (4) anomalous low-level echoes believed to be produced by the boundary surfaces between differentially moistened surface air over adjacent cold and warm water. These observations and those previously reported of sea breeze angles constitute impressive empirical proof of the association between angel echoes and meterologically induced gradients in refractive index. However, it is still not possible to account for the observed angel cross-sections theoretically using “reasonable” gradients of refractive index. The regular observation of elevated blobbly angel layers indicates that conditions are frequently suitable for enchanced forward-scatter beyond the horizon of otherwise line-of-sight electromagnetic waves, Furthermore, the data testify to the feasibility and remarkable foresightedness of Friends's 1939 proposal to use radio reflections for the continuous sounding of air-mass boundaries.

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