The ozone layer is Earth's protection from the sun's harmful ultraviolet radiation. Most of the ozone layer is found in the stratosphere - six to 18 miles above the surface of the planet. Scientists have detected the largest-ever hole in the ozone layer over Antarctica, measuring three times the size of the United States. The hole in this protective layer is blamed on human-made chlorine compounds such as refrigerants and aerosol sprays.

Chlorine is present in the ozone layer in forms that are not ozone harmful. But in early winter, when it is very cold in Antarctica, clouds form in the stratosphere, and these chlorine gases collect on cloud particles and are converted into forms that are extremely ozone destructive. Between 10 and 25 miles above Earth's surface, ultraviolet sunlight over Antarctica triggers the reactions that destroy ozone.