UV radiation monitoring: UV index and 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 dose
Integration of the erythemal UV index, as derived from satellite observations, from sunrise to sunset, with a time step of 10 minutes. The integration takes the cloud cover into account and thus leads to an estimate of the daily erythemal UV dose: the total amount of UV radiation absorbed by the human skin during the day, expressed in kJ/m2.
For this integration, it is necessary to know what the cloud cover was for that day, i.e. how much of the sky was obscured by clouds. And it is necessary to estimate how much these clouds attenuate the UV radiation. The latter is still a matter of investigation: different types of clouds absorb the UV radiation to a different degree.
Since the clouds continously change during a day, cloud cover information would be necessary from minute to minute, which is not practicle. Instead, we use the cloud cover data available once every hour from Meteosat, one of the geostationary satellites in order to compute the UV dose of the previous day. Unfortunately, the relevant cloud cover information for yesterday's UV dose is available only for Europe, so that we currently can compute the daily erythemal UV dose for Europe only. See our erythemal UV dose page.
When not taking cloud cover information into account in the integration, one gets what might be called a "clear-sky UV dose": the maximum UV dose for that day, in the absence of clouds.

The UV dose is compute in time steps of 10 minutes and these values are summed to acquire the total erythemal UV dose for the day. The blue curve shows the UV dose in intervals of 10 minutes for De Bilt (Netherlands) on 1 June 2002, a completely cloud-free day. The red curve shows the cummulative UV dose, with as end value 4.0 kJ/m2.

Graph of the UV dose in intervals of 10 minutes for Augsburg (Germany) on 1 June 2002, showing the effect of clouds passing over the area. The green curve is the clear-sky UV dose, i.e. assuming there are no clouds, while the red curve is the actual UV dose. The blue curve shows the cloud cover fraction, which is given in 1-hour intervals. The clear-sky UV dose for the full day is 4.4 kJ/m2; the actual erythemal UV dose for the full day is 3.7 kJ/m2.

Note:
Along the x-axis in the above two graphs is plotted the local solar time, which is symmetrical around local solar noon, the moment when the Sun is highest in the sky (i.e. in the zenith). The time in UTC is given by:
UTC_time = local_solar_time - longitude/15 For De Bilt the difference is 21 minutes, and for Augsburg it is 43 minutes.