"... an interesting new theory on cloud formation has emerged. In short, it holds that [cosmic rays], which are the high energy particles that arrive at Earth from supernovas, spur cloud formation by ionizing air molecules that act as seeds for water condensation and cloud formation. ... When the sun’s output is high, cosmic rays are prevented from hitting the Earth, and vice versa. So high solar activity corresponds to low cosmic ray flux and therefore lower cloud formation and higher temperatures. "