Atmospheric noise

Atmospheric noise

Atmospheric noise is radio noise caused by natural atmospheric processes, primarily lightning discharges in thunderstorms.

Atmospheric noise is mainly caused by cloud-to-ground flashes as the current is much stronger than for cloud-to-cloud flashes. On a worldwide scale, eight million lightning flashes occur daily. This is about 100 lightning flashes per second.

The sum of all these lightning flashes result in atmospheric noise. It can be observed [Sample of atmospheric noise|url=http://www.ycars.org/EFRA/audio%20files/atmospheric%20noise.mp3] with a radio receiver in the form of a combination of white noise (coming from distant thunderstorms) and impulse noise (coming from a near thunderstorm). The power-sum varies with seasons and nearness of thunderstorm centers.

Although lightning has a broad-spectrum emission, its noise power increases with decreasing frequency. Therefore, at Very Low Frequency and Low frequency, atmospheric noise often dominates, while at High Frequency, man-made noise dominates in rural areas.

From 1960s to 1980s, a worldwide effort was made to measure the atmospheric noise and variations. This has been documented in the CCIR 322 [CCIR Report 322 Noise Variation Parameters|url=http://www.spawar.navy.mil/sti/publications/pubs/td/2813] publication, which is now superseded by ITU P.372 [ITU, Recommendation P.372: Radio Noise|url=http://www.itu.int/rec/R-REC-P.372/en] publication.

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