UV radiation monitoring: UV index and UV dose

UV radiation
monitoring:
UV index
UV dose

Main data product pages:

Erythemal UV index | Daily erythemal UV dose

Introduction to
UV radiation
monitoring

UV radiation

UV & health

UV index

forecasts of
UV index

UV dose

example:
1 June 2002

links

UV radiation
Electromagnetic radiation or "light" is the collective name for all forms of energy that move with the speed of light. There are different "types of light" in the spectrum, depending on their energy, which is related to the wavelength (freqency) of the light: the lower the wavelength, the higher the energy.
The human eye is sensitive for only a part of the spectrum, referred to as "visible light": between 400 and 780 nm (1 nm = 10^-9 meter). The wavelength of the light determines the colour: 400 nm is blue, 700 nm is red.
The part of the spectrum immediately to the left of blue, between 200 and 400 nm is the ultraviolet light (UV). The UV is usually divided into three components, with increasing energy:

UV-A: 320-400 nm
UV-B: 280-320 nm
UV-C: 200-280 nm
Of these UV-B is the most dangerous form, since part of it reaches ground level, where it can affect human health (e.g. cause certain forms of skin cancer) and damage the DNA of flora and fauna. Note that the wavelength of the division between the UV-A and UV-B bands is sometimes set at 315 nm (see note below).
The UV-C energy is potentially more dangerous, but it decreases dramatically as ozone increases, because of the strong absorption in the 200-280 nm wavelength band. The UV-B is also strongly absorbed, but a small fraction reaches the surface. The UV-A is only weakly absorbed by ozone, with some scattering of radiation near the surface.

The curve shows a typical vertical profile of ozone in the midlatitudes of the northern hemisphere: the concentration of ozone as function of altitude. Superimposed on the figure are plots of UV radiation as a function of altitude for UV-A, UV-B and UV-C. The width of the bar indicates the amount of energy as a function of altitude. UV-C is absorbed completely in the stratosphere. Of the global UV radiation at the ground, 94% is UV-A, 6% is UV-B.
figure adapted from Stratospheric Ozone, An Electronic Textbook

Atmospheric ozone thus shields life at the surface from most of the harmful components of the solar UV radiation. Chemical processes in the atmosphere can effect the level of protection provided by the ozone in the upper atmosphere.
Ozone decline in the stratosphere can be caused by:

Chemical processes resulting from the breakdown of CFCs and other ozone depleting gases.
Changes in the stratospheric meteorology, e.g. due to changes in the climate and in trace gases such as nitrous oxide (N2O), water (H2O) and methane (CH4).
Thinning of the atmospheric ozone in the stratosphere leads to elevated levels of UV-B at ground level and increases the risks of DNA damage in living organisms. A 1% decrease in ozone, for example, will lead to an estimated increase of UV-B of about 2%.
It is therefore important to monitor the UV radiation that reaches the ground. One of the tools for this is the UV index.

Note on the UV-A & UV-B wavelength ranges
The wavelength of the devision between UV-A and UV-B varies in the literature and this may lead to some confusion.
The Commission Internationale de l'Éclairage (International Commission on Illumination) uses 280-315 nm as UV-B and 315-400 nm as UV-A.
Other sources put the devision point at 320 nm, as in the above given definition. In particular this is done in medical (dermatological) applications, as well as in cosmetics. Also several text books on UV use 320 as devision point.
To avoid confusion, one could use the following short-hand notation: dUVB for 290-320 nm and dUVA for 320-400 nm, where "d" stands for dermatological, and use UVA and UVB for the 315-nm devision quantities. The use of the prefix "d" is not very common, though.
Within the TEMIS project, the data supplied are the UV index and UV dose, which cover (parts of) both UV-A and UV-B. The precise wavelength range that is relevant for these quantities depends on the action spectrum applied:

UV range relevant for erythema: 280 - 400 nm
UV range relevant for general DNA damage: 256 - 370 nm