Ultraviolet index

The ultraviolet index, or UV index, is an international standard measurement of the strength of the sunburn-producing ultraviolet (UV) radiation at a particular place and time. It is primarily used in daily and hourly forecasts aimed at the general public. The UV index is designed as an open-ended linear scale, directly proportional to the intensity of UV radiation, and adjusting for wavelength based on what causes human skin to sunburn. The purpose of the UV index is to help people effectively protect themselves from UV radiation, which has health benefits in moderation but in excess causes sunburn, skin aging, DNA damage, skin cancer, immunosuppression, and eye damage, such as cataracts. The scale was developed by Canadian scientists in 1992, and then adopted and standardized by the UN's World Health Organization and World Meteorological Organization in 1994. Public health organizations recommend that people protect themselves (for example, by applying sunscreen to the skin and wearing a hat and sunglasses) if they spend substantial time outdoors when the UV index is 3 or higher; see the table below for more detailed recommendations.

Description

The UV index is a linear scale that measures the intensity of UV radiation with respect to sunburn. For example, assuming similar spectral power distributions, radiation with a UV index of 12 is twice as intense as radiation at a UV index of 6. For a wide range of timescales, sunburn in response to controlled UV radiation occurs in proportion to the total number of photons delivered, not varying with the intensity or duration of exposure. Therefore, under similar conditions, a person who develops a sunburn after 30 minutes of exposure to UV index 6 radiation would most likely develop a sunburn of similar severity after 15 minutes of exposure to UV index 12 radiation, since it is twice the intensity but half the duration. This linear scale is unlike other common environmental scales such as decibels or the Richter scale, which are logarithmic (the severity multiplies for each step on the scale, growing exponentially). An index of 0 corresponds to zero UV radiation, as is essentially the case at night. An index of 10 corresponds roughly to midday summer sunlight in the tropics with a clear sky when the UV index was originally designed; now summertime index values in the tens are common for tropical latitudes, mountainous altitudes, areas with ice/water reflectivity and areas with above-average ozone layer depletion. While the UV index can be calculated from a direct measurement of the UV spectral power at a given location, as some inexpensive portable devices are able to approximate, the value given in weather reports is usually a prediction based on a computer model. Although this may be in error (especially when cloud conditions are unexpectedly heavy or light), it is usually within ±1 UV index unit as that which would be measured. When the UV index is presented on a daily basis, it represents UV intensity around the time of solar culmination (when the Sun reaches its highest point during the day), called solar noon, halfway between sunrise and sunset. This typically occurs between 11:30 and 12:30, or between 12:30 and 13:30 in areas where daylight saving time is being observed. Predictions are made by a computer model that accounts for the effects of Sun–Earth distance, solar zenith angle, total ozone amount, tropospheric aerosol optical depth, elevation, snow/ice reflectivity, and cloud transmission, all of which influence the amount of UV radiation at the surface.

Technical definition

The UV index is a number linearly related to the intensity of sunburn-producing UV radiation at a given point on the Earth's surface. It cannot be simply related to the irradiance (measured in W/m2) because the UV of greatest concern occupies a spectrum of wavelengths from 295 to 325 nm, and shorter wavelengths have already been absorbed a great deal when they arrive at the Earth's surface. However, skin damage from sunburn is related to wavelength, the shorter wavelengths being much more damaging. The UV power spectrum (expressed as watts per square meter per nanometer of wavelength) is therefore multiplied by a weighting curve known as the CIE-standard McKinlay–Diffey erythemal action spectrum. There are some older formulas for the spectrum, resulting in differences of up to 2%. The result is integrated over the whole spectrum. This gives a weighted figure called the Diffey-weighted UV irradiance (DUV) or the erythemal dose rate. Since the normalization weight is 1 for wavelengths between 250nm and 298nm, a source of a given DUV irradiance causes roughly as much sunburn as a radiation source emitting those wavelengths at the same intensity, although inaccuracies in the spectrum definition and varying reactions by skin type may mean this relationship does not actually hold. When the index was designed, the typical midday summer sunlight was around 250 mW/m2. Thus, for convenience, the DUV is divided by 25 mW/m2 to produce an index nominally from 0 to 11+, though ozone depletion is now resulting in higher values. To illustrate the spectrum weighting principle, the incident power density in midday summer sunlight is typically 0.6 mW/(m2) at 295 nm, 74 mW/(m2) at 305 nm, and 478 mW/(m2) at 325 nm. (Note the huge absorption that has already taken place in the atmosphere at short wavelengths.) The erythemal weighting factors applied to these figures are 1.0, 0.22, and 0.003 respectively. (Also note the huge increase in sunburn damage caused by the shorter wavelengths; e.g., for the same irradiance, 305 nm is 22% as damaging as 295 nm, and 325 nm is 0.3% as damaging as 295 nm.) Integration of these values using all the intermediate weightings over the full spectral range of 290 nm to 400 nm produces a figure of 264 mW/m2 (the DUV), which is then divided by 25 mW/m2 to give a UV index of 10.6.

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