Solar radiation for any given location and time of year and day is influenced by five factors:
- astronomical;
- geographical (latitude and altitude);
- physical (scattering of solar radiation by atmospheric pollutants/aerosols and absorption mainly by water vapour);
- geometric (solar altitude and azimuth as well as slope steepness and aspect); and
- meteorological (cloudiness, type and thickness of the cloud, reflectivity).
From these five factors, daily estimates of solar radiation for 50 years were calculated at a 1.7 km resolution using equations developed and verified for South Africa (Schulze, 2008; Schulze and Chapman, 2008). The modelling for the intermediate future (2050s) was conducted using five different CMIP3 GCMs (Schulze et al. 2016).
More detail is provided in the section Background: Climate Change Modelling.