Radiation at wavelengths less than about 400 nm is ultraviolet and accounts for about 7% of total solar radiation. When discussing ozone, we are concerned with radiation in the ultraviolet region of the electromagnetic spectrum. In addition to gamma rays and X-rays, which are absorbed high in the atmosphere, ultraviolet (uv) radiation in the atmosphere is divided into three spectra: uva , uvb, and uvc.

Uva falls right below visible light, with wavelengths that vary from 320 to 400 nm. Although it is not absorbed by ozone, uva is the least energetic and the least damaging of all uv radiation.

Uvb radiation, which ranges in wavelength from 280 to 320 nm, is more energetic than uva and thought to be harmful to the biosphere. Fortunately, it exists in lesser amounts and is largely absorbed by ozone.

Uvc, at 200 to 280 nm, which is the most energetic and most damaging but least prevalent of the uv radiation types, is totally absorbed by ozone and normal diatomic oxygen high in the atmosphere.

Ozone is most effective at absorbing radiation at the 250 nm wavelength. In fact, it is 100 times more efficient at 250 nm than it is at 350 nm. After ozone absorbs this shortwave radiation, it reradiates it at generally longer wavelengths which initially goes in all directions. Some is reabsorbed by other atmospheric constituents, some makes it to Earth's surface, and some returns to space. The net effect, however, is an increase in temperature in the upper stratosphere.

Many factors affect the amount of uv radiation that reaches Earth's surface. In addition to the amount of ozone in the stratosphere,the angle of the sun, length of daylight hours, path length of radiation through the atmosphere (all determined by latitude and time of year), solar output, and the type and thickness of clouds are also important factors.

The sun's angle from the vertical or solar zenith angle, is determined by the latitude, time of year, and time of day. In the Northern Hemisphere, the sun shines directly overhead at noon at the Tropic of Cancer on the first day of summer, at the equator on the first day of spring and autumn, and directly overhead at the Tropic of Capricorn on the first day of the winter.

As a point moves farther from the latitude where the sun shines directly overhead, the radiation must travel a longer path through the atmosphere to reach the point.

With a longer path length, there is an increased opportunity for the incoming radiation to be absorbed or reflected back into space by atmospheric constituents. During winter, as you approach the pole, the solar zenith angle increases, and the length of the day decreases. As a result, the total radiation received from the sun at the top of the atmosphere drops off sharply.

During the summer, all latitudes poleward of 23 1/2 experience a smaller solar zenith angle than they do in the winter; however, this angle grows larger as the pole is approached. In summer, a larger sun angle is compensated for by longer day lengths as you travel toward the pole. Therefore, the amount of radiation received at the top of the atmosphere varies little between the equator and about 60 latitude.

Thick clouds absorb uv radiation effectively. However, cumulus clouds can sometimes have the opposite effect as uv radiation is scattered at the edges of these clouds and subsequently reaches Earth's surface. Uv radiation that makes it all the way to Earth's surface can be either reflected or absorbed. Snow is very efficient at reflecting uv radiation, reflecting 85%. Sand reflects 12%, and water, 5%.

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