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The Sun - How the index works

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BBC Weather gives an interpretation of how the sun index works.
BBC Weather looks at ultraviolet radiation and how to interpret the uv index.

The Problem

Most of us know that during the summer months the ultraviolet radiation (also known as UVR) from the sun can damage our skin. Short term this results in sunburn, but long term it is known to cause premature ageing of the skin, and in some cases skin cancer. The speed with which this damage occurs depends on the strength of the ultraviolet radiation but also on the individual skin colour (White skin is damaged far faster than black skin). It is therefore essential that weather forecasters should have a method of warning people of when to expect strong ultraviolet radiation, so people can take steps to protect themselves.

The different types of Skin Colour
Skin specialists recognise seven different skin colours, however it is unlikely that the public could accurately categorise themselves in these, so for simplicity the sun index recognises four categories of skin colour.

1. White skin that burns easily and tends not to tan.
2. White skin that tans easily.
3. Brown skin.
4. Black skin.

Most people will know which of these categories their skin colour fits, however if they are unsure, and feel that it's between two, they should always opt for the paler category (lower number).

What determines the uv index?
The sun index itself depends on two factors:

• The position of the sun in the sky.
• The amounts of cloud cover.

The position of the sun in the sky changes through the day, reaching its highest point around midday. This means that the ultraviolet radiation is strongest at that point in day. The rise and fall is fairly steep:

The peak daily ultraviolet radiation level changes over the year. The strongest being at the Summer solstice (21st June) and the weakest at the Winter solstice (21st December).

Cloud cover however, will greatly affect the uv index. Thick cloud can greatly reduce ultraviolet radiation levels and, surprisingly, certain types of thin cloud can magnify the ultraviolet radiation strength. It is the job of meteorologists to take the documented ultraviolet radiation level for a particular day in the year, and then taking into account the cloud cover forecast, make a prediction of how strong the ultraviolet radiation at ground level will be - this will be the sun index.

When you hear on the weather forecast that the uv index is going to be moderate, for example, how do you interpret this for your own particular skin colour?

Sun index - 1

Sun index - 2
low risk
low risk
low risk
low risk

Sun index - 3

Sun index - 4

Sun index - 5
medium
low
low
low

Sun index - 6

Sun index - 7

Sun index - 8
high
medium
low
low

Sun index - 9

Sun index - 10
very high
high
medium
medium

Sun index - 11
extremely high
high
high
medium
For more information on your skin type and the effects of UV levels, visit the (External) The British Association of Dermatologists website.



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