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Summer's coming, so what exactly is the UV Index?

May 2011

SummerSummer is on its way, and along with the lovely light evenings will come the warnings about sun exposure and ensuring we are protected. More and more forecasts are now giving UV index information on a daily basis, and we will also be bombarded with information on suncream protective factors.

But what exactly is this UV?

Ultraviolet (UV) light is just a small sector of the light that arrives from our sun. Sunlight arrives here in three different forms: infrared or heat; a visible light which we all see and enjoy; and ultraviolet radiation. Without getting too scientific, UV light is named because it consists of electromagnetic waves that have frequencies higher than humans identify as the colour violet.

As with anything scientific, it continues to become more complicated. UV light doesn’t just arrive on our earth from the sun, but arrives in three different forms. There is UVA, UVB and UVC. These rays are defined by their different wavelengths. UVC rays have the shortest wavelengths and are filtered out by the earth’s ozone layer and so are generally disregarded.

With the other two types of ultraviolet rays, UVA penetrates deeper into the skin than UVB and it is UVA that is thought to be the main cause of leathery skin, dark blotches, freckles and other problems caused by over exposure to the sun. However, UVB radiation is the key one that causes the tanning and sunburn effect on our skin. Modern sun creams offer protection against UVA as well as UVB.

UV measurements and the UV index is based on the amount of general skin damaging UV radiation that is expected to reach the earth’s surface at the time when the sun is at its most intense, ie when it is highest in the sky. It is generally based on UVB levels but give a good indication of all the sunlight that is expected to reach the earth at a given time.

Determining UV levels not a straightforward activity. The amount of UV radiation is of course related to the position of the sun, but other variables include the amount of ozone in the stratosphere and also the level of cloud cover. Even this is not as straightforward as it sounds, because while thick cloud can greatly reduce ultraviolet radiation levels, certain types of thin cloud can cause the radiation levels to magnify and strengthen.

The UV index was developed in Canada in 1992 and today it has become an international standard. The level of radiation is indicated by numbers from 0 to 12 – and even higher in the tropics and at especially high locations under clear skies. In the UK the UV Index rarely goes higher than 8 while 9 and 10 are common in the Mediterranean regions. In Western Australia, an index number of 17 has been recorded.

The higher the UV index number, the greater the strength of ultraviolet radiation hitting the earth’s surface and the higher the risk of damage to us.The UV index given in weather forecasts here in the UK can either be simplified as low or high risk, or given its precise measure.

UV index numbers 0 – 2 indicate low risk and pretty well nothing to worry about, however long you are outside. Obviously your skin type and level of general exposure to the sun will play a role here, and you could still experience some redness of the skin after two hours around midday in the summer, but generally 0 – 2 is a very safe level indeed.

Numbers 3 – 5 indicate a medium risk and the general advice is to avoid being in direct sunlight for much more than two hours, although of course as above there are many variables here.

Numbers 6 – 7 indicate high risk and the potential to burn within the hour; numbers 8 – 10 are very high risk when the obvious precautions are needed.

The one warning that everyone should heed is that ultraviolet radiation is not necessarily prevented by cloud cover. It is a very complex science indeed and the simple number index is an excellent start in providing a quick and easy way to inform people of the general strengths of the sun. For more information, two useful websites are:

Met Office’s UV index for the UK:

Met Office’s UV index for the Europe:

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