Cloudy Sky, Location Unknown, 2004 Photograph by Peter Essick Piles of cotton-like cumulus clouds fill the sky on a clear day. Mid-level cumulus clouds form between 6,500 feet (2,000 meters) and about 20,000 feet (6,000 meters). They form when humid air cools enough for water vapor to condense into droplets or ice crystals. A single cloud can hold billions of pounds of water, but may not always produce rain.
Earth Day, celebrated annually on April 22, marks the anniversary of the 1970 birth of the environmental movement. Scientists warn that rising temperatures worldwide could fuel extreme weather—just one of many damaging effects of global warming.