Ultraviolet Radiation (UVR) and the UV Index
Fact sheet on ultraviolet radiation (UVR) and the UV Index
What is UVR?UVR is a form of radiation given out by the sun. Unlike other forms of solar radiation, such as light and heat, UVR cannot be seen or felt.
UVR is the part of the electromagnetic spectrum with wavelengths between 100 and 400 nanometers (nm). It is divided into three types, UVA, UVB and UVC, according to wavelength.
- UVA (longer wavelength) causes premature aging and wrinkling of the skin and is a cause of skin cancer
- UVB (medium wavelength) is more dangerous than UVA and is the major cause of skin cancers, sunburning and cataracts
- UVC (shorter wavelength) is extremely dangerous but does not reach the earth's surface due to absorption in the atmosphere
UVR is also given out by solaria and by other artificial sources such as arc welders and fluorescent, mercury vapour, metal halide and quartz halogen lamps.
Why is UVR dangerous?Exposure to UVR can cause not only sunburn but also lasting damage to skin and eyes. This may result in premature skin ageing, skin cancer and eye disorders such as cataracts, pterygium and cancer of the conjunctiva and eyelid. The effects of UVR are cumulative, so damage builds up even without burning.
How does UVR reach you on the ground?Solar UVR can reach you on the ground from three sources:
- directly from the sun
- scattered from the open sky
- reflected from the environment
Some surfaces reflect large amounts of UVR onto skin and eyes. Snow reflects as much as 80 percent. Other surfaces which reflect large amounts are white paint, light coloured concrete, water and sand.
What factors affect UVR levels?The amount of UVR varies depending on how much of it is absorbed in the atmosphere. Absorption is least and UVR levels are highest when the sun is high in the sky and the sun's rays take the shortest path to earth. Absorption is also affected by the ozone level and the cloud cover. The amount of UVR you receive differs depending on:
- Time of year - in southern areas of Australia, UVR levels are highest in midsummer (January). In northern areas, UVR levels are high all year
- Time of day - the sun is higher in the sky in the middle of the day. In Australia, the highest levels of UVR are between 10am and 2pm (11am and 3pm daylight saving time)
- Position on earth - UVR levels are higher closer to the equator, for example, in northern areas of Australia
- Altitude - the thinner atmosphere at high altitude filters out less UVR. There is about a 10 percent increase in UVR for each 1000m increase in altitude above sea level
- Ozone - ozone absorbs some UVR that would otherwise reach the earth. While ozone levels vary during the day and with the season, other factors such as sun height and changes in cloud cover appear to have more influence on levels of UVR
- Cloud cover – UVR levels are highest under cloudless skies and heavy cloud can result in less UVR. However, light or thin cloud cover does not reduce UVR and may even increase the levels due to scattering
What is the UV Index?The Ultraviolet (UV) Index is an international standard measurement of how strong UVR from the sun is at a particular place on a particular day. The UV Index ranges from 0 to 11+ and the higher the number the greater the risk of skin and eye damage.
How can the UV index help me to protect myself?The UV Index provides useful information that can help you plan outdoor activities in ways that minimise exposure to the sun's rays. The UV Alert, which is issued when the UV Index forecast reaches 3 or higher, shows the time of the day when it is essential to protect yourself.
Where can I find out about the UV index?The daily UV Index forecast and UV Alert can be found in the weather pages of local papers, and on the websites of the Australian Bureau of Meterology and the Australian Radiation Protection and Nuclear Safety Agency.
Information on the UV Index is also broadcast on television channels and radio stations.