UV index and UV dose: surface elevation correction
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Surface elevation correctionThe amount of UV radiation received by someone at the Earth's surface is affected by the local surface elevation: the higher the surface, the more UV radiation. For this reason the UV index and UV dose calculation applies a 5% increase of the incident UV irradiance per km surface elevation above sea level:f(H) = 1 + 0.05·Hwhere the surface elevation
H(in km w.r.t. sealevel) is determined from the GMTED2010 global digital elevation model, which is averaged to the UV grid resolution of 0.25°×0.25°.
The averaging of the GMTED2010 data over the UV grid cell provides also a standard deviation, which is propagated to the UV index and UV dose error estimates.
If you would like to adjust the elevation correction from the grid cell average evelation
H_gridto a specific elevation
H_new, the above equation shows how, since
f(H)is a multiplication factor in the UV index or UV dose calculation:UV_new = UV_grid * f(H_new) / f(H_grid)with
UV_gridthe UV index or UV dose value from a gridded TEMIS UV data file.
GMTED2010 based surface elevation data (left) and standard deviation (right)
at the UV grid resolution of 0.25°×0.25°.
Note on the percentage increase per kmThe TEMIS UV dose service provides the UV dose for three action spectra. The increase of the UV radiation with 5% per km elevation mentioned above is an estimate made for the erythemal UV radiation, i.e. for the UV radiation covering both the UV-A and UV-B wavelenghts, based on the regular (Raleigh) optical effects in the atmosphere for those wavelengths.
The other two action spectra, vitamin-D and DNA-damage, stress the shorter UV-B wavelengths much more, and for those data products the percentage increase might well be 10% or more. On the other hand the Raleigh scattering is less for shorter wavelengths.
In addition, in the presence of clouds, the scattering by clouds dominates of the UV radiation is dominant and is the height dependence of the UV under the clouds small.
For practical reasons we have chosen to use one percentage for all TEMIS UV data products.
- For some more info on the GMTED2010 data, its source and description, and for download of the data averaged to different resolutions, see this page.
last modified: 3 December 2020
data product contact: Jos van Geffen & Michiel van Weele & Ronald van der A
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