The Index system set-up principally to make warnings a lot clearer, easier to understand, and consistent. In addition, the sun index brought our service in line with others internationally.

The Index system is quite simple. It ranges from 1 to 10; 1 being a very low risk, whereas 10 would be a very high risk. This scale makes it immediately clear whether you are in danger, and they can be used on quite a local level. In fact, air pollution and sun index ratings are available in our five-day forecasts for UK towns and cities.

Sun Index - Yellow Triangles
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. 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 sun 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. Find out more information on Sun Index.

Pollen - Green Symbols
In the pollen season, we also have an index showing your risk from tree, grass and weed pollen. The season varies each year as a result of the weather in spring and early summer, but generally runs from late April until the end of August.

The pollen count is worked out by counting the number of pollen grains in a sample of air. When the data arrives at the BBC Weather Centre, we convert it into an index to make it easier to understand, and consistent with our other services.

Air Pollution - Purple Symbols
The air pollution index is new to the BBC Weather Centre. We previously used to have a separate reading for each of the different pollutions in the atmosphere, however this wasn't as user friendly because you had to look at each pollutant in turn to ascertain the extent of the problem in your area. Now, however we have combined the pollutants to give you one figure which shows the risk you are facing overall.

Key to symbols

1 - 3 (Low)
Effects are unlikely to be noticed, even by those sensitive to air pollution.

4 - 5 (Moderate)
Sensitive people may notice mild effects but are unlikely to need action.

6 - 7 (High)
Sensitive people may notice significant effects and may need action to reduce or avoid them (for example by spending less time outdoors).

8 - 10 (Very High)
Effects are as for high, but symptoms may be worse, or may affect more people.