Uv index how is it measured

This results in a number that usually ranges from 0 where there is no sun light to the mid teens. This value is the UV Index. Thus, the UV Index for the example city would be:.

Currently, the computation of the UV Index does not include the effects of variable surface reflection e. ANd for an extreme level of detail - try the Climate Prediction Center's pages. Toggle navigation Home. Rather, it is computed using forecasted ozone levels, a computer model that relates ozone levels to UV incidence on the ground, forecasted cloud amounts, and the elevation of the forecast cities.

As an example, assume the following UV levels for each wavelength are predicted for a given location these are totally made up numbers, and not even the ratios represent reality : Wavelength Incidence nm 10 nm 20 nm 50 This information is then weighted according to how human skin responds to each wavelength; it is more important to protect people from wavelengths that harm skin than from wavelengths that do not damage people's skin.

The calculation done by some other nations also includes ground observations. The calculation starts with measurements of current total ozone amounts over the entire globe, obtained via two satellites operated by the National Oceanic and Atmospheric Administration. These data are used to produce a forecast of stratospheric ozone levels for the next day at many points across the country. A computer model uses the ozone forecast and the incident angle of sunlight at each point to calculate the strength of UV radiation at ground level.

Sunlight angle is determined by latitude, day of year, and time of day solar noon. The strength of UV radiation is calculated for several wavelengths between and nm, the full spectrum of UVB nm and UVA nm radiation.

Ozone in the atmosphere absorbs attenuates shorter UV wavelengths more strongly than longer wavelengths. The strength of ground-level UV radiation differs significantly across the UV spectrum. As an example, UV strengths for a point might be calculated as shown in the table below. These are hypothetical values. The UV index is calculated based on four factors: Advertisement.

The thickness of the ozone layer over your city detected using satellites The cloud cover over your city clouds block UV radiation to varying degrees The time of year in winter, UV radiation is lower than in the summer because of the sun's angle The elevation of your city higher elevations get more UV radiation. UV Intensity maps. Cite This!

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