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Ultraviolet index

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Title: Ultraviolet index  
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Collection: Atmospheric Radiation, Hazard Scales, Meteorological Indices, Ozone Depletion, Skin Care, Sun Tanning
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Ultraviolet index

The ultraviolet index or UV Index is an international standard World Meteorological Organization in 1994. It is primarily used in daily forecasts aimed at the general public, and is increasingly available as an hourly forecast as well.

The UV Index is designed as an open-ended linear scale, directly proportional to the intensity of UV radiation that causes sunburn on human skin. For example, if a light-skinned individual (without sunscreen or a suntan) begins to sunburn in 30 minutes at UV Index 6, then that individual should expect to sunburn in about 15 minutes at UV Index 12 – twice the UV, twice as fast.

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, sunglasses) if they spend substantial time outdoors when the UV Index is 3 or higher; see the table below for more-detailed recommendations.


  • Description 1
  • Technical definition 2
  • History 3
  • How to use the index 4
    • Cautionary notes 4.1
  • See also 5
  • Notes 6
  • External links 7


The UV Index is a linear scale, with higher values representing a greater risk of sunburn (which is correlated with other health risks) due to UV exposure. 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 [6] though other scientists dispute readings higher than 26.[7]

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.[8]

Typical variation of UV Index by time of day and time of year, based on FastRT UV Calculator[9]

When the UV Index is presented on a daily basis, it represents UV intensity around the sun's highest point in 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 elevation and distance, stratospheric ozone, cloud conditions, air pollutants, surface albedo, and ground altitude, all of which influence the amount of UV radiation at the surface.[2] The calculations are weighted in favor of the UV wavelengths to which human skin is most sensitive, according to the CIE-standard McKinlay-Diffey erythemal action spectrum.[10][11] The resulting UV Index cannot be expressed in pure physical units, but is a good indicator of likely sunburn damage.

Because the index scale is linear (and not logarithmic, as is often the case when measuring things such as brightness or sound level), it is reasonable to assume that one hour of exposure at index 5 is approximately equivalent to a half-hour at index 10, although other factors like the body's ability to repair damage over a given time period could detract from the validity of this assumption.

Technical definition

Sunburn effect (as measured by the UV Index) is the product of the sunlight power spectrum (radiation intensity) and the erythemal action spectrum (skin sensitivity) across the range of UV wavelengths.[10][11]

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 wavelength from 295 to 325 nm, and shorter wavelengths have already been absorbed a great deal when they arrive at the earth's surface. Skin damage from sunburn, however, is related to wavelength, the shorter wavelengths being much more damaging. The UV power spectrum (strictly expressed in watts per square metre per nanometre of wavelength) is therefore multiplied by a weighting curve known as the erythemal action spectrum, and the result integrated over the whole spectrum. This gave Canadian scientists a weighted figure (sometimes called Diffey-weighted UV irradiance, or DUV, or erythemal dose rate) typically around 250 mW/m2 in midday summer sunlight. So, they arbitrarily divided by 25 mW/m2 to generate a convenient index value,[12][13] essentially a scale of 0 to 11+ (though ozone depletion is now resulting in higher values, as mentioned above).

To illustrate the spectrum weighting principle, the incident power density in midday summer sunlight is typically 0.6 mW/(nm m2) at 295 nm, 74 mW/(nm m2) at 305 nm, and 478 mW/(nm 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[12] 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.[13]


After sporadic attempts by various meteorologists to define a "sunburn index", and amid growing concern about ozone depletion,

  • UV Awareness — hourly UV Index forecasts for locations around the world
  • FastRT UV Calculator — Enter any date, time, location, local conditions; compute "UV dose rate" of type "Skin burn"; and divide result by 25 to obtain the UV Index.
  • World Health Organization UV Radiation program — including links to many UV Index reporting sites
  • ESA/TEMIS UV Index forecast and archives — daily data for Europe and the world
  • SunWise by the US EPA — background information and UV Index forecasts
  • USDA UV-B Monitoring and Research Program: Erythemal Radiation — years of historical data
  • FMI Global UV Index service — global daily maximum maps and hourly clear-sky forecast graphs, plus observations from Finland
  • Australian National UV Index Forecast

External links

  1. ^ Hanneman K.K., Cooper K.D., Baron E.D. (2006), Ultraviolet immunosuppression: mechanisms and consequences. Dermatologic Clinics, 24 (1): 19–25.
  2. ^ a b Fioletov V., Kerr J., Fergusson A. (2010), The UV Index: Definition, Distribution and Factors Affecting It. Canadian Journal of Public Health, 101 (4): I5–I9.
  3. ^ Gies P. et al (2011), UVR Emissions from Solaria in Australia and Implications for the Regulation Process. Photochemistry and Photobiology, 87 (1): 184–190.
  4. ^ Gerber B. et al (2002), Ultraviolet Emission Spectra of Sunbeds. Photochemistry and Photobiology, 76 (6): 664–668.
  5. ^ Hornung, R.L. et al (2003), Tanning facility use: are we exceeding Food and Drug Administration limits?. Journal of the American Academy of Dermatology, 49 (4): 655–661.
  6. ^ Cabrol N.A., Feister U., Häder D.-P., Piazena H., Grin E.A., Klein A. (2014), Record solar UV irradiance in the tropical Andes. Frontiers in Environmental Science, 2 (19).
  7. ^ McKenzie R.L., Bernhard G., Madronich S., Zaratti F. (2015), Comment on “Record solar UV irradiance in the tropical Andes, by Cabrol et al”. Frontiers in Environmental Science, 3 (26).
  8. ^ "UV Index: Is It Validated?" NOAA. 2006.
  9. ^ Engelsen O. and Kylling A. (2005), Fast simulation tool for ultraviolet radiation at the Earth's surface. Optical Engineering, 44 (4): 041012–041012-7.
  10. ^ a b McKinlay A.F. and Diffey B.L. (1987), A reference action spectrum for ultraviolet induced erythema in human skin. CIE Journal, 6 (1): 17–22.
  11. ^ a b "UV Spectral Irradiances & Erythemal Action Spectrum". NOAA. 2006.
  12. ^ a b "How UV Index Is Calculated". EPA SunWise. 2012.
  13. ^ a b "How Is the UV Index Calculated?" Smithsonian Institution. Accessed August 20, 2007. (This source contains some numerical errors.)
  14. ^ Kerr J.B., McElroy C.T., Tarasick D.W., Wardle D.I. (1994), The Canadian Ozone Watch and UV-B advisory programs. Proceedings of the Quadrennial Ozone Symposium 1992, 794–797.
  15. ^ "Environment Canada's UV Index Celebrates Ten Years" (Press release).  
  16. ^ "Report of the WMO Meeting of Experts on UV-B Measurements, Data Quality and Standardization of UV Indices, 1994" (Global Atmosphere Watch, 95). World Meteorological Organization. 1995.
  17. ^ a b c "Global Solar UV Index: A Practical Guide" (PDF).  
  18. ^ "Blazing World Record: Strongest UV Rays Measured in South America". 
  19. ^ "UV Alert". EPA SunWise. 2011.
  20. ^ "SunSmart UV Alert". Cancer Council Australia. 2014.
  21. ^ "Ozone awards".  
  22. ^ a b "UV Index Scale". EPA Sunwise. 2014.
  23. ^ Hu L.W. et al (2010), Diurnal Variations in Solar Ultraviolet Radiation on Horizontal and Vertical Plane. Iranian Journal of Public Health, 39 (3): 70–81.
  24. ^ a b Dresbach S.H. and Brown W. (2008). "Ultraviolet Radiation" (PDF). Ohioline Fact Sheet Series. CDFS-199-08. Ohio State University Extension. 
  25. ^ Berking C. (2005), The role of ultraviolet irradiation in malignant melanoma. Hautarzt, 56 (7): 687–696.


See also

  • The intensity of UV radiation reaching the surface of the earth depends on the angle of the sun in the sky. Each day, the sun achieves its highest angle (highest intensity, shortest shadows) at solar noon, which only approximately corresponds to 12:00 on clocks. This is because of the differences between solar time and local time in a given time zone. In general, UV risk is high when the sun is directly enough overhead that people's shadows are shorter than their height.[22]
  • Likewise, UV intensity can be higher or lower for surfaces at different angles to the horizontal. For example, if people are walking or standing outdoors, UV exposure to the eyes and vertical surfaces of skin, such as the face, can actually be more severe when the sun is lower,[23] such as the end of a summer's day, or winter afternoons on a ski trail. This is partly a consequence of the fact that the measurement equipment upon which the index is based is a flat horizontal surface.
  • UV intensity can nearly double with reflection from snow[17] or other bright surfaces like water, sand, or concrete.[24]
  • The recommendations given are for average adults with lightly tan skin. Those with darker skin are more likely to withstand greater sun exposure, while extra precautions are needed for children, seniors, particularly fair-skinned adults, and those who have greater sun sensitivity for medical reasons[24] or from UV exposure in previous days. (The skin's recovery from UV radiation generally takes two days or more to run its course.)
  • Because of the way the UV Index is calculated, it technically expresses the risk of developing sunburn, which is caused mostly by UVB radiation. However, UVA radiation also causes damage (photoaging, melanoma[25]). Under some conditions, including most tanning beds, the UVA level may be disproportionately higher than described by the UV Index. The use of broad-spectrum (UVA/UVB) sunscreen can help address this concern.

When interpreting the UV Index and recommendations, be aware that:

Cautionary notes

UV Index Media graphic color Risk of harm from unprotected sun exposure, for the average adult Recommended protection
0–2.9 Green "Low" Wear sunglasses on bright days; use sunscreen if there is snow on the ground, which reflects UV radiation, or if you have particularly fair skin.
3–5.9 Yellow "Moderate" Take precautions, such as covering up, if you will be outside. Stay in shade near midday when the sun is strongest.
6–7.9 Orange "High" Cover the body with sun protective clothing, use SPF 30+ sunscreen, wear a hat, reduce time in the sun within three hours of solar noon, and wear sunglasses. (Australian slogan: Slip-Slop-Slap-Seek-Slide.)
8–10.9 Red "Very high" Wear SPF 30+ sunscreen, a shirt, sunglasses, and a wide-brimmed hat. Do not stay in the sun for too long.
11+ Violet "Extreme" Take all precautions: Wear SPF 30+ sunscreen, a long-sleeved shirt and trousers, sunglasses, and a very broad hat. Avoid the sun within three hours of solar noon.

When the day's predicted UV Index is within various numerical ranges, the recommendations for protection are as follows:[17][22]

How to use the index

In 2007, the United Nations honored UV Index inventors Kerr, McElroy and Wardle with the Innovators Award for their far-reaching work on reducing public health risks from UV radiation.[21] In the same year, a survey among meteorologists ranked the development of the UV Index as #11 for The Weather Channel's 100 Biggest Weather Moments.

In 2005, the United States[19] and Australia[20] launched the UV Alert. While the two countries have different baseline UV intensity requirements before issuing an alert, their common goal is to raise awareness of the dangers of over-exposure to the sun on days with intense UV radiation.

On Dec. 29, 2003, a world-record UV index of 43.3 was detected at Bolivia's Licancabur volcano.[18]

In the United States, the WHO standards officially replaced the original US standards in 2004. [17] gradually replaced the inconsistent regional versions, specifying not only a uniform calculation method (the Canadian definition) but also standard colors and graphics for visual media.[16]

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