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Global Solar Ultraviolet Index

Author: Vanessa Ngan, Staff Writer, 2003. Updated by Dr Richard McKenzie, Emeritus Researcher – Atmospheric Radiation, NIWA, New Zealand. DermNet NZ Editor-in-Chief: Adjunct Assoc. Prof. Amanda Oakley, Dermatologist, Hamilton, New Zealand. September 2016.


What is the global solar ultraviolet index?

The global solar ultraviolet index (UVI) describes the level of solar ultraviolet radiation (UVR) at the Earth's surface and ranges from 1 to 11+. The higher the UVI, the greater the potential for damage to the skin and eyes, and the shorter the time before skin damage (manifested as skin reddening, or erythema).

The National Institute of Water and Atmospheric Research (NIWA) displays predicted and measured UVI in New Zealand regions.

  • Free smartphone apps using GPS to display local UVI include:
    • uv2Day for New Zealand, Australia, the South Pacific region, and AntarcticaAndroid and iPhone.
    • GlobalUV for any location in the world: Android and iPhone

Global solar ultraviolet index

What affects the ultraviolet index?

The UVI is affected by:

  • The angle of the sun: the higher the sun in the sky, the higher the UVI. This means that UVR levels vary with the time of the day and the time of year.
  • Latitude: the closer to equatorial regions, the higher the UVI.
  • Cloud cover: UVI is highest on a clear day, but even with cloud cover, UVI can be high. And with scattered cloud, it can be even higher if the sun is unobscured by clouds, because of reflection from cloud edges.
  • Altitude: the thinner atmosphere at higher altitudes means less UVR is absorbed. In clean air, UVI increases by about 5% per cent per km increase in altitude. In polluted areas, the gradient can be much greater. 
  • Ozone: ozone absorbs some UVR that would otherwise reach the Earth's surface. Annual mean ozone amount is 300 DU. In NZ, ozone levels vary seasonally, with a maximum in spring of around 400 DU and a minimum in autumn of around 250 DU. Variations from day to day can exceed 10%.
  • Reflection: UVR is reflected by different surfaces; fresh snow can reflect as much as 80% of UVR, sand can reflect about 15%, and sea foam about 25%.

Peak UVI in New Zealand is about 40% more than at similar latitudes in the Northern hemisphere summer. This is due to Earth being closer to the sun during the Southern hemisphere summer (a 7% effect), our lower ozone amounts in the Southern Hemisphere summer (~ 10% effect), and much clearer skies in the southern hemisphere (~ 20%).

What do the UVI numbers mean?

Levels 1 and 2: In the green band, levels one and two, the UV level is low. Low protection is needed and people can safely stay outside.

Levels 3 to 5: The yellow band, numbers three to five, indicates moderate UV levels. Protection is required when spending long periods outside. Kiwis will be advised to follow the Cancer Society's slip, slop, slap and wrap behaviours.

Levels 6 and 7: Protection is essential at levels six and seven, represented by an amber band. Kiwis will be advised to follow the Cancer Society's slip, slop, slap and wrap behaviours.

Levels 8 to 10: When UV levels are in the very high red band, eight to ten, it's recommended people seek shade between 11 am and 4 pm, slip, slop, slap and wrap and make sure they reapply sunscreen at least every two hours.

Level 11+: Levels of 11 or higher, shown in a purple band, are regarded as extreme. Reschedule activities for the early morning and evening. Full protection is essential between 11 am and 4 pm.

How does ultraviolet radiation damage the skin?

Ultraviolet radiation (UVR) is the most important preventable risk factor for skin cancer. New Zealand has the highest rates of melanoma in the world. Nearly 300 people die from skin cancer (mostly from melanoma) and more than 50,000 cases of melanoma and non-melanoma skin cancer are diagnosed in New Zealand each year. The cost to the health system is believed to be more than NZ$50 million annually.


  • Damages skin (sunburn, premature skin ageing and cancer)
  • Damages eyes (keratitis and cataracts)
  • Suppresses immunity in the skin and internally
  • Predisposes to bacterial and viral skin infection
  • Exacerbates pre-existing actinic keratoses
  • Prevents innate immunity from rejecting skin tumours

Sun protection

Whenever the UVI is over 3, and especially when UVI levels are high, sun protection should include:

  • Avoidance of exposure to the sun around the solar noon (1.30 pm in summer, in New Zealand)
  • Indoor activities or seeking shade when outdoors
  • Wearing sun-protective clothing
  • Putting on sunglasses
  • Applying broad spectrum high protection SPF 50+ sunscreen

Avoid artificial sources of UVR, such as sunbeds, at all times. 

Further information

The global solar UVI was developed by the World Health Organisation (WHO) in partnership with the World Meteorological Organisation (WMO), the United Nations Environment Program (UNEP), the International Agency for Cancer Research (IARC), the International Commission on Non-Ionizing Radiation Protection (ICNIRP) and collaborating centres including the Cancer Society of New Zealand, the Health Sponsorship Council's SunSmart brand and the National Institute of Water and Atmospheric Research (NIWA).

One UVI Unit is equivalent to 25mW/m2 UVR reaching the Earth's surface. The UVI forecasts usually report at least the daily maximum UVR levels averaged over a 30-minute time period, assuming there is no cloud cover and other modifying factors.

The Global Solar UVI is formulated using the International Commission on Illumination (CIE) reference action spectrum for UV-induced erythema on the human skin. The UVI is defined by the formula:

UVI formula

Refer to Intersun for an explanation.



  • McKenzie, R. L., Johnston, P. V., Smale, D., Bodhaine, B. A. & Madronich, S. Altitude effects on UV spectral irradiance deduced from measurements at Lauder, New Zealand, and at Mauna Loa Observatory, Hawaii. Journal of Geophysical Research-Atmospheres106, 22845-22860, doi:10.1029/2001jd900135 (2001) Global Solar UV Index, A practical Guide (WHO 2002)
  • McKenzie, R. L., Bodeker, G. E., Scott, G. & Slusser, J. Geographical differences in erythemally-weighted UV measured at mid-latitude USDA sites. Photochemical & Photobiological Sciences 5, 343-352 (2006).
  • Cancer Society of NZ
  • Balancing risks and benefits of UV radiation
  • Richard McKenzie, NIWA Lauder. Calculation of UVI for smartphone apps PDF file

On DermNet

Other websites

Australians and New Zealanders can download Smartphone app, uv2Day, for Android and iPhoneGlobalUV for any location in the world: Android and iPhone 

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