How does UVR intensity vary during the day?

This graph shows the UV Index for Melbourne on a cloud free day (solid line) and a day when there was heavy cloud and rain, which reduced the UVR (dotted line). There is also a computer model, which is a prediction of the UV levels for a clear sky day (dashed line). The model is calculated using a measured ozone value for the day. Unusual cloud conditions and variation in ozone levels may occasionally cause the measured UV-Index to exceed the model prediction.

These graphs of average monthly UV-Indexes for some widely separated Australian capital cities show that regions closer to the equator (further north) have significantly higher levels of solar UVR especially during the winter months.

This map shows the latitudes of some widely separated Australian capital cities.

How does the atmosphere affect solar UVR?

The following graph compares the solar UVR spectrum from outside the atmosphere with the spectrum at the earth's surface, as measured by ARPANSA in Melbourne. The effects of absorption by the atmosphere can be clearly seen. The earth's atmosphere absorbs most of the UVB and some of the UVA.