Uv Index Calculator

The UV Index is not a raw irradiance reading but a biologically weighted one. Solar UV between roughly 250 and 400 nm is multiplied by the CIE erythemal action spectrum, which weights each wavelength by how efficiently it reddens human skin, integrated across the band, and then divided by 25 milliwatts per square meter to yield a single dimensionless number. That divisor is why an index of 1 corresponds to 25 mW per square meter of erythemally effective irradiance. The dominant physical driver is the solar zenith angle, since a lower sun forces the beam through a longer atmospheric path and more ozone absorption, which is why the estimate here is built from latitude and solar declination. Two corrections follow: elevation adds roughly 6 to 8 percent per 1,000 meters as overlying atmosphere thins, and cloud cover subtracts, though thin or broken cloud may cut UV by only 10 to 25 percent and scattering off cloud edges can briefly raise it above clear-sky values. Reported values are bucketed as low below 3, moderate 3 to 5, high 6 to 7, very high 8 to 10, and extreme at 11 or above. Burn time scales inversely with the index and is modified by Fitzpatrick skin phototype, with type I skin reaching a minimal erythema dose several times faster than type IV. The main limitations are that this model assumes solar noon and clear standard-atmosphere conditions and does not account for total column ozone, aerosol loading, or surface albedo - fresh snow reflects up to 80 percent of incident UV and water transmits roughly 40 percent to a depth of one meter. SPF-extended exposure times are also theoretical ceilings, since laboratory SPF testing applies 2 mg per square centimeter, considerably more product than most people use.

The idea of publishing a daily public UV forecast originated in Canada. Scientists at Environment Canada, working in the aftermath of the Antarctic ozone hole discovery reported by Joseph Farman, Brian Gardiner and Jonathan Shanklin of the British Antarctic Survey in 1985, launched the world's first national UV Index forecast in 1992. That original Canadian scale placed a clear summer noon in Toronto near 10 and was expressed in different units than today's version, and other countries soon issued their own incompatible scales, which made international comparison and travel advice difficult. Standardization followed quickly. The World Health Organization, together with the World Meteorological Organization, the United Nations Environment Programme and the International Commission on Non-Ionizing Radiation Protection, harmonized the definition in the mid-1990s and published the consolidated reference, Global Solar UV Index: A Practical Guide, in 2002. That document fixed the erythemal weighting, the 25 mW per square meter normalization, the open-ended scale, and the standard color coding from green through purple that national weather services still use. The underlying biology came earlier: the erythemal action spectrum was refined by the Commission Internationale de l'Eclairage during the 1980s, and dermatologist Thomas Fitzpatrick introduced his skin phototype classification at Harvard in 1975, originally to guide phototherapy dosing rather than sunscreen advice. In the United States the EPA and National Weather Service began issuing UV Index forecasts in 1994.