So far, 2011 has proved a year destined for the tornado record books.
Nearly 1,200 tornadoes have swarmed the United States this year, according to preliminary numbers from the National Oceanic and Atmospheric Administration (NOAA).
Four of these storms have been rated at the highest tornado strength, an EF-5.
The death toll from these tornadoes has likely topped 500, a number not seen since 1953.
But why has this year seen so many and such devastating twisters? Scientists point to
several large-scale climate factors, some of which have been at work behind the scenes since winter. And at least some of the mind-boggling tornado numbers, believe it or not,
can be chalked up to humans — there are more of us around to see them.