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Electric charge released by radiation from cold war nuclear bomb tests may have changed rainfall patterns thousands of kilometres away, in the UK.

Rain is formed in warm clouds when small droplets of condensation collide and merge to form larger and larger droplets, until these fall down as rain drops. Electric charge in the atmosphere, including that formed during the radioactive decay of strontium-90 and other radioactive material used in nuclear weapons, can increase the likelihood that these small droplets will collide to form rain.

There was a major increase in atmospheric electric charge in the middle of the 20th century following a wave of cold war atomic bomb tests, often carried out in Nevada, US, Kazakhstan in the Soviet Union and in the Pacific. Giles Harrison at the University of Reading, UK, and his colleagues analysed historical records of this spike in atmospheric radioactivity and electric charge, as well as the historical information on daily cloud density and daily rainfall. The data was collected between 1962 and 1964 at two weather stations in the UK 鈥� in London and Shetland.

The team found that there was 24 per cent more rainfall on average on the days with higher electric charge from radioactivity, which is thought to have originated mainly from nuclear bomb test detonations that were performed by the US and the Soviet Union in the 1950s and early 1960s.

Understanding how electric charge in the atmosphere can influence cloud density and rainfall could inform the development of strategies to manipulate weather, says Harrison.

鈥淭here have been various schemes proposed for putting charge into clouds to modify their properties but I just don鈥檛 think we鈥檝e had experimental results really to guide that,鈥� says Harrison. 鈥淥ne of the values of this piece of work is it gives you an idea of what amount of charge you鈥檇 need.鈥�

Of course, we wouldn鈥檛 want to use nuclear bombs to generate this sort of charge, says Harrison. 鈥淏ut you could conceivably use aircraft that could release charge,鈥� he says, adding that research efforts in this direction are still at a very early stage.