Ultraviolet radiation (UV) is defined as that part of the electromanetic spectrum between the visible region and the x-ray region. While about 50% of the Sun's energy is concentrated in the visible portion of the spectrum, about 40% is in the infrared and 10% in the UV.

All of the major atmospheric gases, nitrogen, oxygen, carbon doxoide, water vapor, and ozone absorb in the UV wavelengths, however, ozone is by far the most important absorber. That's the good news. The bad news is that chloroflorocarbons (CFCs), especially chlorine, like to gobble-up ozone. Chlorine has a strong chemical attraction for ozone. In the presence of sunlight, chlorine breaks down ozone into molecular oxygen and a substance called chlorine monoxide. The chlorine in other CFCs as well as the newly-formed chlorine monoxide, both are available to attack the ozone. These nasty CFCs can remain in the atmosphere for decades. Much of chlorine in the stratosphere has originated from manmade chemicals, and some of it is from natural sources.

Because UV radiation is more energetic than visible radiation, increases in the amount of UV reaching the surface may produce harmful effects in most all life forms. It's been estimated by the United Nations Environment Program, that a 25% rise in the incidence of non melanoma skin cancers could occur if ozone levels in the stratosphere were to drop by about 10%.

The place where the ozone depletion is most obvious is over Antarctica. The Antarctic ozone hole develops with the return of spring sunlight to the south polar stratosphere. Ozone, which has stayed fairly constant during the winter dark period, begins to decline in early September and continues to decline until early October. This is also when the UV levels are the highest and when the south polar vortex is fully developed. The polar vortex is simply an area where winds circulate around the poles forming a "wall" which extends from the stratosphere towards the surface, preventing air from the troposhere and stratosphere from mixing. The region defined by the ozone hole roughly matches the coldest regions of the Antarctic stratosphere, where low-temperature polar stratospheric clouds form. In late October the vortex begins to break up, and by December it has completely dissipated. As a result, ozone is mixed in from adjacent areas thereby increasing ozone amounts to pre-spring levels.

In the Arctic, the temperatures are not as cold as in the Antarctic, and so the polar vortex is not as well defined and is more easily disturbed. Thus, some mixing occurs in different levels of the atmosphere and ozone levels do not drop off as noticeably as they do in the Antarctic - there is more of an ozone pore rather than a hole. Therefore, UV levels in the Arctic, whether in spring or summer, are not as high as they are in the Antarctic.

The intensity of UV radiation is primarily a function of solar elevation, time of year, latitude, clouds, and elevation above sea level. It is lower during the winter than during the summer, and it is higher in the middle of the day than in the morning or evening. At about 40 degrees north latitude, the intensity of the UV radiation is approximately six times greater in summer than in winter.

In the U.S. and in the mid latitudes, the Sun is much higher in the sky during summer than it is during spring in Antarctica, when the UV levels are greatest. Because the Sun's rays are more direct, UV levels are higher in parts of the U.S. in summer than they are in Antarctica in September or October. There's much more ozone over the U.S., but there's also much more incoming UV radiation. Additionally, over the southwestern U.S., fewer clouds are available to thwart some of the UV from reaching the surface. Even though we in the U.S. may receive more UV than Antarctica, the ozone hole there is a recent feature, and life forms have not had sufficient time to adapt to the increases in UV. How this increasing UV affects important food chain links, such as plankton and krill, remains to be seen.