Wind power has long been touted as a major energy resource, but for decades no one knew how much energy it could actually yield. Then three years ago Stanford University atmospheric scientists Cristina Archer and Mark Jacobson did a detailed calculation based on known patterns of air motion. Using a conservative approach, they counted only the energy that could be generated from winds blowing over land at an altitude of 80 meters, the approximate height of a typical modern wind turbine. Under perfect conditions, the total would be 72 trillion watts.