Factors affecting UV radiation levels
1 Season and time of day
The single most important factor is the height of the sun in the sky – the higher the sun, the higher the levels of UV radiation. The height of the sun varies with season and time of day.
  • Season - UV radiation levels are less intense in winter than in summer because the sun is lower in the sky in winter, and the path of radiation through the atmosphere is longer.
  • Time of day - UV radiation levels are more intense at solar noon than in the morning or afternoon because the sun is at its highest point at solar noon.
A graph of daily UV radiation shows that it is most intense at solar noon. We get 60% of total UV radiation between 11am and 3pm

2 Geographic location
Your latitude also contributes to UV radiation levels. The closer you are to the equator the higher the UV radiation levels. This is why UV radiation levels in Australia and New Zealand are higher than those in Europe and most of North America. Within Australia and New Zealand, UV radiation levels are consistently high in summer regardless of latitude, but in winter they vary greatly.

Altitude also affects UV radiation levels. UV radiation levels increase by around 4% with every 300 metres of altitude.

3 Surrounding environment
Different surfaces reflect UV radiation to greater or lesser degree, and so environments which contain highly reflective surfaces have high indirect UV radiation levels. See the table below for the reflectivity of different surfaces:

4 Other factors
Heavy cloud can reduce UV radiation levels to less than 5% of that under clear skies. Scattered cloud has a variable effect - levels rise and fall as clouds pass in front of the sun.

Decreases in stratospheric ozone (the “ozone layer”) increase UV radiation levels, and a sustained decline has the potential to significantly increase skin cancer rates in the long term.

Air pollutants and other particles in the atmosphere, such as dust, can reduce UV radiation levels.