ScienceNOW (8/5, Berardelli) reported, "Most of Earth's clouds get their start in deep space. That's the surprising conclusion from a team of researchers who argue that interstellar cosmic rays collide with water molecules in our atmosphere to form overcast skies." A new study by Henrik Svensmark of the Technical University of Denmark found that over the past 22 years the five strongest coronal mass ejections from the sun, which would block cosmic rays from reaching the Earth, corresponded to a 7% decrease in cloud cover. "Svensmark argues that the findings suggest a link between cosmic rays and climate change. Because clouds...reflect light from the sun, fewer clouds would mean a warmer Earth." However, Jón Egill Kristjánsson of the University of Oslo noted that there has only been a "slightly upward trend" at most in cosmic rays. "That would mean either no increase in cloud formation or a slight increase--neither of which would warm the world."
Further reading if you're interested:
Study shows strong evidence that cloud changes may exacerbate global warming
*Random fact about me: I have always been fascinated with climate/weather, and for a while I dreamed of working for NOAA. It's still my back up plan (I have many plans - A, B, C, & D).