Learning outcomes:
By the end of this activity you should know:
- there are two components (direct and diffuse) that make up the global solar UV radiation
- the diffuse component is as important as the direct component
- the amount of diffuse UV is controlled mainly by the solar zenith angle and cloud cover.
Global solar UV radiation measured on the earth’s surface, also known as total UV radiation, is made up of two components: direct and diffuse radiation.
Direct solar UV radiation is the radiation that is received at an object’s surface directly from the sun, i.e. arriving with the sun’s beam, causing shadows. Personal exposure to this component of radiation can be minimised by blocking its path using hats, tree shade and other shade structures.
Diffuse solar UV radiation is the UV radiation that has been modified by atmospheric and environmental scattering before reaching the object’s surface. This radiation is incident on the object’s surface from all directions and is difficult to decrease using shade structures as it depends on the total available sky view.
The relative amounts of direct and diffuse solar UV components to global solar UV radiation depend on many factors such as solar zenith angle (SZA) and cloud cover.
The following presentation shows the direct UV exposure, diffuse UV exposure and total sky images recorded during a summer day in Toowoomba.
Questions
- Calculate the ratio of the diffuse to global UV at the following times: 10:50, 12:55, 13:50 and 17:00.
- What is the predominant component for high solar zenith angles?
- What is the predominant component when percentage cover of clouds is high?
- What is the percentage of diffuse exposure when the percentage cover of clouds is low?