A late summer sunset over the Rio Grande River Valley and Albuquerque, New Mexico. Hurley has watched these sunsets from the Sandia Mountains, and says: "...the warmth of the continent drives the rich moist air north from the tropics over the desert. After a day of sunlight the air heats up and in the evening great clouds billow upward to the freezing atmosphere. After the sun goes down, it lights these clouds from below, often casting their shadows on the aprons of ice crystals spreading out above them."