Cosmic rays, cloudy days
Having examined 50 years of solar radiation measurments, Giles Harrison and David Stephenson of the University of Reading, UK, found that cosmic rays can increase the chances of a cloudy day by nearly 20%. Normally, solar cosmic rays are about 90% protons (hydrogen) and 9% alpha particles (helium). However, during a solar flare - exlosion on the Sun that happens when energy stored in twisted magnetic fields, typically above sunspots, is suddenly released - there is a significant burst of radiation across the spectrum (radio, x-rays, gamma rays), flares are classified according to x-ray brightness (1-8 Angstroms). So, the bright side of a cloudy day is that you’re shielded from radiation. Further, this may be a clue to explaining some of the mysterious climate changes our planet has experienced in the past (Big Ice Age, Little Ice Age).
(Proceedings of the Royal Society A, DOI: 10.1098/rspa.2005.1628).
Cosmic rays linked to cloudy days, New Scientist.com
SpaceWeather.com