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

Source: New Scientist