Moon effect on lightning

Belief that the moon has an effect on the weather is present for centuries. It is probably a result of the well-known effect of the moon's gravitational force on the ocean tides. It was only in the beginning of 1800s that scientists try to prove it. Most of them, however, didn't find any conclusive evidence. In earlier 1900s, a lunar tide on the surface air pressure was demonstrated, but its amplitude was found to be so small to produce any significant weather change. After 1950, several evidence that the lunar phases have an effect on precipitation and on ice nucleus concentrations were reported, giving a renew interest on the subject. In order to explain such evidence, some authors attributed the lunar influence to lunar modulation of meteoritic dust reaching the Earth, although others attributed this to gravitational tides or, even, a result of the lunar effect on the long wave circulation in the troposphere. In spite of such evidence, the prevailing view at the present time is that no appreciable effect on weather should be expected. In particular, the effect of moon on thunderstorm activity was a common believed of most navigators from the 1500s to 1800s centuries. Differently of precipitation, there is only three studies (published by the same author) trying to find the lunar influence on thunderstorm activity, all of them based on thunderstorm days for two relatively small periods (1930-1933 and 1942-1965) collected in 108 stations in the United States. The studies found an increase in the thunderstorm activity two days after full moon and attributed it to the lunar modulation of the earth's magnetic field in the geomagnetic tail and/or a unknown effect on the lower atmosphere caused by the aligning of Sun, Earth and moon. In this report, thunderstorm days collected in three stations in the South and Southeast regions of Brazil during 55 years and lightning data detected in the Southeast region in the last 15 years are analysed to investigate a possible dependence on lunar phases. As far as we known, this is the first study to use lightning data for this purpose. The results of the analysis show evidence of a decrease in the thunderstorm and lightning activity in the third quarter phase. We also found an increase in the lightning activity in the new phase, not observed in the thunderstorm data and less pronounced that the decrease in the quarter. The lack of a symmetric result with respect to the moon suggests that the effect is probably not due to gravitational tides and may be related to moon interaction with the Earth's magnetic field or other unknown effect. Since now lightning data are available for large regions and reasonable long periods, we encourage other researchers to search for similar effect in other countries.