Atmospheric noise is radio noise caused by natural atmospheric processes, primarily lightning discharges in thunderstorms. On a worldwide scale, there are about 40 lightning flashes per second – ≈3.5 million lightning discharges per day.[1]
In 1925, AT&T Bell Laboratories started investigating the sources of noise in its transatlantic radio telephone service.[2]
Karl Jansky, a 22-year-old researcher, undertook the task. By 1930, a radio antenna for a wavelength of 14.6 meters was constructed in Holmdel, NJ, to measure the noise in all directions. Jansky recognized three sources of radio noise.[3] The first (and strongest) source was local thunderstorms. The second source was weaker noise from more distant thunderstorms. The third source was a still weaker hiss that turned out to be galactic noise from the center of the Milky Way. Jansky's research made him the father of radio astronomy.[4]
In early 1950s, a mathematical model of the impact of lightning and thunderstorms on broadcasting was published by S. V. C. Aiya [5]
Atmospheric noise is radio noise caused by natural atmospheric processes, primarily lightning discharges in thunderstorms. It is mainly caused by cloud-to-ground flashes as the current is much stronger than that of cloud-to-cloud flashes.[citation needed] On a worldwide scale, 3.5 million lightning flashes occur daily. That means there are about 40 lightning flashes per second.[1]
The sum of all these lightning flashes results in atmospheric noise. It can be observed,[6] 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 urban areas.
From 1960s to 1980s, a worldwide effort was made to measure the atmospheric noise and variations. Results have been documented in CCIR Report 322.[7][8] CCIR 322 provided seasonal world maps showing the expected values of the atmospheric noise figure Fa at 1 MHz during four hour blocks of the day. Another set of charts relates the Fa at 1 MHz to other frequencies. CCIR Report 322 has been superseded by ITU P.372[9] publication.
Atmospheric noise and variation is also used to generate high quality random numbers.[10] Random numbers have crucial applications in the security domain.[11]
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