The Ultraviolet Index
The strength of UVR in the environment is measured by the UV Index (UVI). This is a scientific measurement used internationally to measure UVR levels. The higher the UVI number the greater the UVR level. The UVI is promoted internationally by the World Health Organization. “A Practical Guide” to the UVI can be downloaded from http://www.who.int/uv/publications/globalindex/en/.
A UVI level below 3 is low and a UVI level above 10 is extremely high. Whenever the UVI level is 3 and above, sun protection is required because this level of UVR is intense enough to damage the skin and contribute to the risk of skin cancer.
In New Zealand UVI levels vary widely throughout the year.
Over most of New Zealand the UVI is usually over 3:
• Between September and April (especially between 10am and 4pm)
• In winter at high altitude and in snow.
The UVI level is influenced by: the latitude, the amount of ozone in the atmosphere, the amount of cloud cover, reflection from surfaces such as concrete, the time of year and time of day.
NIWA developed the following UVI Table to illustrate mean and peak UVI levels (taken at solar noon) throughout the year at five New Zealand centres.
Table 1: Mean UVI Levels throughout the Year at Five New Zealand Centres
Table 2: Peak UVI Levels (Solar Noon) Throughout the Year at Five New Zealand Centres
Find out the UVI in your area
To help you work out how best to protect yourself from the sun you can find out the UVI in your region plus your own personal recommendations of appropriate sun protection measures
To find the UVI for your area go to:
o NIWA website - click on your town/city and find out the UVI and what time during the day you need to protect yourself from the sun. The diagrams show UV levels at specific times of the day and give a predicted maximum UV level for sunny and cloudy skies.
o SunSmart website – this provides a map of New Zealand. Click on your town/city for the forecast of the maximum UVI for the day and then find what sun protection is recommended based on your skin type and the time you will be in the sun
See NIWA's report A Climatology of UVI For New Zealand.
