Rockfalls happen about 60 to 70 times per year in Yosemite, but usually they have an obvious cause. A winter storm has blown through, or there was an earthquake. But some, like the Happy Isles incident, happen on beautiful, clear summer days for no apparent reason. Now a pair of scientists says they know what’s behind these rockfalls—it’s the warm, sunny weather itself.
http://www.smithsonianmag.com/science-nature/why-rockfalls-happen-beautiful-days-yosemite-180958581/
