Long-term variation of atmospheric radio noise on 27 kHz generated in South America during the southern summer

Measurements of the integrated field strength of the atmospheric radio-noise on 27 kHz at the Observatory of Kuhlungsborn in the period 1952-89 have been analyzed as a contribution to the long-term variation of thunderstorm activity in South America during the southern summer. In accordance with the hitherto known climatic trends, the long-term variation includes a downward (1952-74) and a stronger upward (1974-89) period. The time evolution of the mean change of the noise level shows a decrease during the last 12 years which may be regarded as a first indication of changes of the noise-generating thunderstorm activity as a response of the tropical meteorology to the deforestation in the Amazon region. For the year-to-year variation of the noise level a mean value of 7.5 percent and a maximum of 20 percent were determined.