David Fahey of the National Oceanic and Atmospheric Administration's (NOAA) Aeronomy Laboratory in Boulder, Colo., explains.

The severe depletion of stratospheric ozone during the winter in Antarctica is known as the "ozone hole." A significant decrease appeared first over Antarctica because atmospheric conditions there increase the effectiveness of reactive halogen gases containing chlorine and bromine that destroy ozone. Formation of the Antarctic ozone hole requires not only these ozone-depleting chemicals but also air temperatures low enough to form polar stratospheric clouds (PSCs).

Ozone-depleting chemicals are produced in the stratosphere from halogen source gases emitted at the earths surface. These source gases, such as chlorofluorocarbons (CFC-11 and CFC-12, for example), are manufactured and released in the troposphere by human activities. Source gases exist in comparable abundances throughout the stratosphere in both hemispheres even though most of the emissions occur in the Northern Hemisphere. The amounts are comparable because most source gases have no important natural removal processes in the lower atmosphere, allowing winds and warm-air convection to redistribute and mix the gases efficiently throughout the troposphere. These well-mixed source gases enter the stratosphere primarily from the upper tropical troposphere. Atmospheric air motions then transport the gases farther upward and toward the poles in both hemispheres. Once in the stratosphere, the source gases--which alone do not destroy ozone--chemically degrade to form ozone-depleting chemicals, such as chlorine and chlorine monoxide.

The severe ozone destruction represented by the ozone hole requires that low temperatures be present in polar regions over a range of stratospheric altitudes, over large geographical regions, and for extended periods. Low temperatures are important because they allow polar stratospheric clouds (PSCs) to form. Reactions on the surfaces of the cloud particles initiate a remarkable increase in the concentrations of the most reactive ozone-depleting chemicals, which in turn elevates the rate of ozone depletion. Temperatures are lowest in the stratosphere over both polar regions during the winter months. The temperatures are low enough for PSCs to form for nearly the entire Antarctic winter but usually only for part of every Arctic winter. Thus, the chemical depletion of ozone in the Arctic is usually less than that in the Antarctic.

PSC particles grow large enough and are numerous enough that cloudlike features can be observed from the ground under certain conditions, particularly when the sun is near the horizon (see image). PSCs often occur near mountain ranges in polar regions because the motion of air over the mountains can cause local cooling of stratospheric air. The formation of PSCs has been recognized for many years from ground-based observations, but scientists did not realize the full geographical and altitude extent of PSCs in both polar regions until PSCs were observed by a satellite instrument in the late 1970s. In addition, the role of PSCs in forming ozone-depleting gases was not known until after the discovery of the Antarctic ozone hole in 1985. Our understanding of the role of PSCs developed from laboratory studies, computer modeling, and direct sampling of PSC particles and reactive chlorine gases (such as chlorine monoxide) in the polar stratospheric regions.

A general thinning of the ozone layer has indeed occurred over the past two decades. The most reactive ozone-depleting substances are also present in the stratosphere at lower latitudes outside of winter polar regions but in much smaller quantities. Most of the chlorine and bromine from the source gases remains in so-called reservoir substances that are much less reactive towards ozone. These smaller quantities of reactive substances also deplete ozone. The stratospheric ozone layer has been diminishing gradually since 1980 and now is about 3 percent lower on average around the globe.