Wall Street Journal: Drought and heat generated by an urban landscape might help explain the powerful tornado that tore through downtown Atlanta exactly a year ago, ripping holes in the roof of the cavernous Georgia Dome stadium and leaving about $100 million in damage in its wake. A new study, conducted by researchers at Purdue University and the University of Georgia, suggests that pockets of rain during a drought interacted with dry air masses, heightening the initial thunderstorm that spawned the …