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Radars, Weather Explanations, Weather Radar
Asset ID: 5395
|Title: What is Anomalous Propagation?|
Concept: Sometimes atmospheric conditiosn send the beam inrto the ground or otherwise corrupt the return.
Detail: A radar beam normally rises above the earth as it travels further from the antenna. But there are
certain unusual atmospheric conditions which cause the beam to bend in strange ways,
including downward. On some occasions, the beam, or portions of it, can even be
deflected downward and strike the earth at distances 100 miles or more from the radar.
These conditions generally occur during intense inversions, or when a layer of moist air is
present over the radar station. The results can often be a bit weird and confusing to the
untrained radar user. Chicago radars have frequently seen the shoreline of the northern
end of Lake Michigan, over 200 miles away. Miami radars have occasionally picked up
Cuba, and most radars, particularly during the night after thunderstorms have passed
through, will see random patches of echoes appear and dissolve as the beam is bent this
way and that by complex atmospheric patterns. A skilled meteorologist can distinguish
precipitation from anomalous propagation by monitoring the character of the signal as
well as by noting the shape and characteristics of the "AP";, as it is often called.
The modern NEXRAD radar processing tends to remove most - but not all - AP, so care is still required when
interpreting a display.