High Quality Content by WIKIPEDIA articles! A Radio Atmospheric signal or Sferic (sometimes also spelled "Spheric") is a broadband electromagnetic impulse that occurs as a result of natural atmospheric lightning discharges. Sferics may propagate from their lightning source without major attenuation in the Earth-Ionosphere waveguide, and can be received thousands of kilometers from their source. On a time-domain plot, a sferic may appear as a single high-amplitude spike in the time-domain data. On a spectrogram, a sferic appears as a vertical stripe (reflecting its broadband and impulsive nature) that may extend from a few kHz to several tens of kHz, depending on atmospheric conditions. Sferics received from about two thousand kilometres distance or greater have their frequencies slightly offset in time, producing tweeks.

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