Our Index Systems

The Index system set-up principally to make warnings 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 originally ranged from one to ten; one being a very low risk, and ten a very high risk. This scale was changed in May 2005 to make it more immediately clear whether you are in danger. The new index system has a three point scale: low risk, moderate risk and high risk.

Air pollution and sun index ratings are available in our five-day forecasts for UK towns and cities, and the pollen index is available at a regional level.

Sun 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. 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
In the pollen season, we 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.

The pollen index can be used to warn sufferers in advance that they might need to increase their medication or stay inside at times when they will be affected.

Air Pollution
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

Low
Effects are unlikely to be noticed, even by those sensitive to air pollution.

Moderate
Sensitive people may notice mild effects but are unlikely to need to take any action.

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

The forecast is provided by a complex chemistry model for 24 pollutants for the next 5 days. The output from the model provides the forecast for five main pollutants (sulphur dioxide, carbon monoxide, nitrogen dioxide, ozone and PM10), which in turn is combined to form the single index used on BBC Weather.

During the summer, the index is dominated by ozone and during the winter it is likely to be dominated by PM10. The bandings within the pollution index are set by DEFRA and the Department of Health, who in turn must follow EU directives on air quality.