Here are the best numbers I've found for the % of low clouds due to Cosmic Rays:

“… cosmic rays. These high-energy particles originate in outer space and in solar flares, and can have a small but significant effect on the weather, increasing the chances of an overcast day by nearly 20 per cent.
Giles Harrison and David Stephenson from the University of Reading, UK, examined 50 years of solar radiation measurements from sites all over the country, enabling them to calculate daily changes in cloudiness. By comparing this data with neutron counts - a measure of cosmic ray activity - for the same period, the scientists have shown an unambiguous link between cosmic rays and clouds (Proceedings of the Royal Society A, DOI: 10.1098/rspa.2005.1628).
"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.”

http://www.newscientist.com/article....mg18925365.700







Erich