Atmospheric noise

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.^[2] On a worldwide scale, 3.5 million lightning flashes occur daily. That means there are about 40 lightning flashes per second.^[3]

The sum of all these lightning flashes results in atmospheric noise. It can be easily heard with any AM radio or SSB receiver tuned to an unused frequency.^[4] The heard static is a combination of white noise (cumulative of distant thunderstorms) and impulse noise (from relatively nearby thunderstorms, if any). The power-sum varies with seasons and nearness of thunderstorm centers. It can be seen as random speckles on an old analog TV set dialed to an empty channel.

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.

In 1925, AT&T Bell Laboratories started investigating the sources of noise in its transatlantic radio telephone service.^{[5](pp 402–408)}

Then a 22 year-old researcher, K.G. Jansky undertook the task in 1928. 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:^{[5](pp 404–405)}

1. The first (and strongest) source was local thunderstorms.
2. The second source was weaker noise from more distant thunderstorms.
3. 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".^[5](p 406)

In early 1950s, S.V.C. Aiya published a mathematical model of the cumulative effects of lightning and thunderstorms on broadcasting.^[6]

From 1960s to 1980s, a worldwide effort was made to measure the atmospheric noise and variations. Results have been documented in CCIR Report 322.^[1][7] CCIR 322 provided seasonal world maps showing the expected values of the atmospheric noise figure F_a at 1 MHz during four hour blocks of the day. Another set of charts relates the F_a at 1 MHz to other frequencies. CCIR Report 322 has been superseded by publication of ITU R-RE-P.372.^[8]