UV Index: What is it?
The UV Index is a next day
forecast of the amount of skin damaging UV radiation expected to reach the earth's surface
at the time when the sun is highest in the sky (solar noon). The amount of UV radiation
reaching the surface is primarily related to the elevation of the sun in the sky, the
amount of ozone in the stratosphere, and the amount of clouds present. The UV Index can
range from 0 (when it is night time) to 15 or 16 (in the tropics at high elevations under
clear skies). UV radiation is greatest when the sun is highest in the sky and rapidly
decreases as the sun approaches the horizon (Figure 1).
The higher the UV Index, the greater the dose rate of skin damaging (and eye
damaging) UV radiation. Consequently, the higher the UV Index, the smaller the time it
takes before skin damage occurs (Figure 2).
There are two prices to pay for overexposure
to UV radiation: a severe sun burn following an intense short term overexposure, and the
more serious skin cancers developing after long term overexposure. Melanoma,
the more deadly of the two types of skin cancer occurs when the patient has been subjected
to several intense short term overexposures. Non-melanoma skin cancers, which are almost
100% curable, occur in people who are overexposed for very long periods of time, like
construction workers, farmers, or fishermen. Long term overexposure to UV radiation has
been linked to the formation of cataracts in the eyes as well.