Uv index how is it calculated

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.

At each wavelength, multiply the actual incoming radiation level by the weighting: Wavelength Incidence Weight Result nm 10 15 nm 20 5 nm 50 3 These weighted irradiances are summed up, or integrated, over the to nm range resulting in a value representing the total effect a given day's UV radiation will have on skin. For our example, the total is If we assume that the example location is at 1 kilometer in elevation, and that there will be broken clouds, then the calculation is: x 1.

JavaScript appears to be disabled on this computer. Please click here to see any active alerts. The U. National Weather Service calculates the UV Index using a computer model that relates the ground-level strength of solar ultraviolet UV radiation to forecasted stratospheric ozone concentration, forecasted cloud amounts, and elevation of the ground.

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.

The UV index is a value calculated each day that helps you to know how much UV radiation will reach you if you go outside. By knowing the index value for the day, you can take appropriate precautions. These precautions help you avoid sunburn in the short term and skin cancer in the long term. The UV index value ranges between 0 and 10, with zero being minimal UV exposure risk and 10 being maximal UV exposure risk.

This page lists the proper precautions to take for the different index values. As established by the inventors of the UV Index, the strength of UV at each wavelength from to nm is summed to represent the overall impact UV radiation has on human skin.

Account for how clouds and elevation interfere with UV. It is also known that UV radiation is absorbed by clouds, which reduces the intensity of UV that hits Earth's surface. Choose the scenario. To continue the example we are doing, lets choose an elevation of 5 kilometers which is the elevation of La Rinconada, Peru - one of the highest cities in the world [5] X Research source and say that scattered clouds are present. You then multiply the total UV effect calculated earlier by the elevation and cloud adjustment.

This calculation would look like the following: x 1. Finish the calculation. To end the calculation, you have to divide the value of UV from the step above by the number 25 this is another part determined by the inventors of the UV Index , and round to the number you find to the closest whole number.

According to the EPA, the calculation results a number between 0 and 15 or so. For the scenario we decided, the UV Index would be: Apply the scale. From the calculated value, read a UV Index scale and adhere to the warnings associated with the value yikes, 15 is very dangerous! Include your email address to get a message when this question is answered.

When determining the true weights for UV strength at each wavelength the weights used in the instructions above are hypothetical use the McKinlay-Diffey erythema action spectrum.

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