Ultraviolet radiation is a form of electromagnetic energy. The various forms of energy, or radiation, are classified according to wavelength, measured in nanometres (nm). One nanometre is a millionth of a millimetre. The shorter the wavelength, the more energetic the radiation. In order of decreasing energy, the principal forms of radiation are gamma rays, X rays, ultraviolet (UV) radiation, visible light, infrared (IR) radiation, microwaves and radio waves. Most energy from the Sun is in the form of visible and UV radiation.
There are three categories of UV radiation:
- UV-A, between 320 and 400 nm
- UV-B, between 280 and 320 nm
- UV-C, between 200 and 280 nm
Generally, the shorter the wavelength, the more biologically damaging UV radiation can be if it reaches the Earth in sufficient quantities. UV-A is the least damaging (longest wavelength) form of UV radiation and reaches the Earth in greatest quantity. Most UV-A rays pass right through the ozone layer in the stratosphere. UV-B radiation can be very harmful. Fortunately, most of the Sun's UV-B radiation is absorbed by ozone in the stratosphere. UV-C radiation is potentially the most damaging because it is very energetic. Fortunately, all UV-C is absorbed by oxygen and ozone in the stratosphere and never reaches the Earth's surface.
Although the ozone layer is the one constant defence against UV penetration, several other factors can effect the amount of UV radiation arriving at the Earth's surface. The Sun’s rays are the most intense near the equator where they impact the Earth’s surface at the most direct angle. At higher latitudes, the Sun is lower in the sky during daylight hours and the strength of radiation is weaker. Furthermore, during the winter months, the Sun's rays strike at a more oblique angle than they do in the summer. This means that all solar radiation travels a longer path through the atmosphere to reach the Earth, and is therefore less intense. Daily changes in the angle of the Sun influence the amount of UV radiation that passes through the atmosphere. When the Sun is low in the sky, its rays must travel a greater distance through the atmosphere and may be scattered and absorbed by water vapour and other atmospheric components. The greatest amount of UV reaches the Earth around midday when the Sun is at its highest point.
Other influences on UV radiation include altitude, cloud cover, air pollution and land type. On the top of a mountain the air is thinner and cleaner. Consequently more UV reaches there than at lower elevations. Clouds can have a marked impact on the amount of UV radiation that reaches the Earth's surface. Thick clouds block more UV than thin cloud cover. Like clouds, urban smog can reduce the amount of UV radiation reaching the Earth. Finally, incoming UV radiation is reflected from most surfaces. Snow reflects up to 85%, whilst other land types reflect much less. Water reflects only 5%. Reflected UV can damage people, plants, and animals just as direct UV does.