IT is not your ordinary hole. Stretching six to 12 miles into the stratosphere, this hole is not a good one - it means part of the sky is missing - in large part because of man-made chemicals.The hole is the gap in the ozone layer which protects Earth from the harmful effects of the sun's rays. The hole has been appearing over Antarctica every spring for the last six years. This year the hole is one of the worst on record as the ozone levels are severely depleted. And the hole is displaying some strange activity. About two weeks ago it stretched in a long elliptical shape from Antarctica across the South Pole to the lower reaches of South America, including Punta Arenas, a Chilean community of nearly 100,000 residents. "It's quite unusual," says Tom Clarkson, manager of the Atmospheric Physics and Chemistry Group at the New Zealand Meteorological Service in Wellington. The extreme elongated shape is the widest the hole has stretched. The main reason is the steady increase in chlorine, which rises by 4 to 5 percent per year. Normally, chlorine is a volatile chemical and dissipates in industrial and household uses. However, chlorine is also used in making very stable chemicals called chlorofluorocarbons (CFCs). CFCs are used in refrigeration, insulation, and many forms of manufacturing. Because CFCs are so stable, the molecules do not degenerate but gradually filter to the upper atmosphere, where they are bombarded by the sun's ultraviolet rays. The sun's radiation tears the chlorine atom from the CFC molecule. This chlorine is free to react with ozone directly as well as with nitrogen oxides (the atmosphere is 60 percent nitrogen), which lock up the chlorine atom in a non-ozone-depleting molecule. The chlorine, however, is ravenous. One atom of chlorine can destroy 100,000 atoms of ozone. (Halons - used in firefighting - can be even deadlier to ozone.) The impact of the chlorine is noticed in Antarctica this time of year. In the Southern Hemisphere's winter, planetary waves transport the chlorine to high altitudes. The chlorine atoms build up in the antarctic stratosphere. Also during the antarctic winter, clouds form in the atmosphere, which is colder than minus 80 degrees C. The chlorine molecules react with the particles in clouds, releasing the bonded chlorine. Once dawn comes, the chlorine atoms become unstable and begin destroying ozone. The ozone-depleted air is surrounded by a high-speed jet stream, known as the "polar night jet," which whips around the antarctic continent six to 30 miles above the ground. As the long winter night envelops Antarctica, the jet stream increases its velocity and isolates the air mass over the continent. Once spring arrives, the jet stream slows down and becomes more easily distorted. Waves of warming air buffet the vortex. "Inevitably it gets bashed apart as it gets slower," says Paul Lehmann, senior physicist with the ozone science unit of the Australian Bureau of Meteorology. Once that happens, the jet stream no longer confines the air mass and the ozone-depleted air moves out. The collapse of the hole is not a sudden event. At the top of the stratosphere warm air with fresh ozone and nitrogen oxides enters the hole. The hole starts to fill in, and by the time it actually breaks up in December, some of the ozone will be replenished. Pieces of the hole break off and may drift over the oceans and land areas. Within the hole, the ozone is depleted, permitting ultraviolet radiation to increase dramatically. "You can have massive changes in a short period of time," says Colin Roy, head of the radiation dosimetry section at the Australian Radiation Laboratory in Yallambie, Victoria. In 1987, Dr. Roy measured a 20 percent increase in ultraviolet radiation over Melbourne when ozone levels fell sharply. The measurement helped confirm the link between depleted upper-atmosphere ozone levels and ultraviolet readings. Dr. Lehmann is concerned that the antarctic ozone hole will break down earlier than usual this year because of increased planetary wave activity. "The sooner it breaks down, the more depleted air that breaks out [escapes]," he says. However, Dr. Clarkson of New Zealand says he sees no indication of an early breakdown. Over the next few years there is an added problem: the explosion of the Mount Pinatubo volcano in the Philippines. Volcanoes expel chlorine as well as sulfur dioxide, which mixes with water to form sulphuric acid. The sulphuric acid acts similarly to the clouds in the antarctic stratosphere in liberating chlorine from its bonded form. After El Chichon exploded in Mexico in April 1982, the ozone layer was reduced by 5 to 10 percent. Since the chlorine levels are now 35 percent higher than the 1982 explosion, Lehmann estimates the effect could be much greater this time. In addition, Mount Pinatubo has ejected two to three times the amount of material as El Chichon. Ozone depletion is not unique to the Antarctic. Scientists are discovering that the ozone layer is depleting in the northern half of the globe, over populated areas. On Oct. 15, British scientists announced a $23.8 million study of the ozone layer in the Northern Hemisphere. Joe Farman, who discovered the antarctic ozone hole, warns that 30 percent of the ozone over Europe could be depleted by the year 2000, when the industrial nations have committed themselves to eliminate the use of CFCs. The scientists are not likely to find a giant ozone hole similar to the antarctic phenomenon because the Northern Hemisphere has more planetary waves caused when air currents hit high mountain ranges. The arctic regions are also warmer than the Antarctic. The warmer air means the temperature gradients are not as steep, so there is no circumpolar wind to trap the ozone-depleted air. However, both hemispheres suffer from midwinter depletion of ozone. This event takes place at the higher latitudes. No one is sure yet why this happens. Scientists hope to better understand the chemical processes after analyzing data received from the recently launched US Upper Atmosphere Research Satellite. Scientists are also working on computer models to help in predicting the stability of the antarctic ozone hole. However, Lehmann says it's likely the models will only be accurate to within a week. The scientists will have plenty of time to try to get it right. The ozone hole is likely to be around until well after the turn of the century.