Australians are now widely aware of the hazards associated with prolonged exposure to ultraviolet radiation. Forecasting the intensity of UVB at ground level must take into account information on the time, date, latitude, cloud, altitude, haze and ozone concentrations. The Bureau of Meteorology worked with the Cooperative Research Centre for Southern Hemisphere Meteorology to develop a special global computer model which predicts ozone concentration, then calculates the intensity of UV radiation expected to reach the ground at any point in Australia. Forecasters then apply a correction factor for the amount of cloud expected at each location around midday. Both the clear-sky and the cloud affected values are included in the forecasts.
Cloudy days deceive many people into thinking that the danger of UV radiation is minimal.
Cloud affects the strength of radiation reaching the ground in complex ways. Most clouds block some UV radiation, but the degree of protection depends on the type and amount of cloud. Some clouds can actually increase the UV intensity at points on the ground by reflecting and refracting the sun's rays. Another complicating factor is the patchiness of cloud cover. People can be caught unawares when a small break in an overcast deck of clouds can allow a brief burst of intense radiation to reach the ground.