The electrifying image took social media by storm: As of Aug. 14, Greg McCown's photo had garnered more than 1,000 Facebook likes and more than 3,600 retweets. There's a reason why it went viral: Those types of sightings are incredibly rare. Although 100 lighting bolts strike the Earth every second, the chances of them flashing near a rainbow are slim, said Randall Cerveny, a professor of meteorology at Arizona State University in Tucson. "Usually, you don't get those two things to line up at the same time," Cerveny said, adding that a desert area like Tucson is more likely...