Since the 1960s, the U.S. Air Force has operated the Defense Meteorological Satellite Program (DMSP) series of operational polar-orbiting meteorological satellite systems. These satellites obtain environmental data that are processed to provide graphical weather images and specialized weather products. Polar satellites also provide data used to monitor environmental phenomena, such as ozone depletion and drought conditions, as well as data sets that are used by researchers for a variety of studies such as climate monitoring.