* Astronomy

Cloudy grey weather is sometimes caused by cosmic rays.

New evidence that events in outer space affect the weather and climate of Earth has been revealed in a study by meteorologists at the University of Reading published in the Proceedings of the Royal Society on Wednesday 18 January, 2006.

In their paper ‘Empirical evidence for a non-linear effect of galactic cosmic rays on clouds’, Drs Giles Harrison and David Stephenson suggest that cosmic rays have a significant effect on the Earth’s lower atmosphere – particularly on levels of cloudiness.

The Reading meteorologists discovered that the chance of an overcast day decreases by 20% on days with low cosmic ray fluxes. The effect is strongest when low cosmic ray fluxes occur – this is often associated with solar flares, but may also result from changes outside the solar system.

"Back in 1959, Edward Ney suggested that variations in cosmic rays, which are charged particles mostly originating outside the solar system, could affect our weather. This research now provides strong evidence supporting Ney’s suggestion, which effectively links atmospheric and space science. As well as the influence of weather, our evidence shows a small yet statistically significant effect of cosmic rays on daily cloudiness. This suggests that cosmic rays are an additional external source of climate variability that should be considered when modelling past and future climate" - Dr Giles Harrison.

To detect changes in the atmosphere from cosmic rays, Harrison and Stephenson used solar radiation measurements made by meteorological stations. They conducted a careful analysis of the UK archives of daily solar radiation observations from 1951-2004 and compared them with neutron counter cosmic ray measurements taken at Climax, Colorado, between 1951 and 2000.

"The odds of a cloudy day increase by around 20 per cent when the cosmic ray flux is high" - Giles Harrison.

This amounts to a few extra days of cloudiness per year.

When cosmic rays hit the atmosphere they produce charged particles which seem encourage the growth of cloud droplets. Compared with greenhouse gases the effect of cosmic rays on climate is small. But it could help explain some of the more mysterious changes in climate Earth has experienced in the past.

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