ABSTRACT

Solar radiation that arrives at the earth’s surface having come directly from the sun is dened as direct normal irradiance (DNI). Even if the sky is clear, DNI is smaller than would be measured at the top of the earth’s atmosphere because DNI has undergone scattering (by molecules and aerosols) and absorption (by gases and aerosols) within the earth’s atmosphere. If clouds are between the sun and the observer, and they are optically thick, then no direct normal irradiance reaches the earth’s surface. The global horizontal irradiance (GHI) observed at the surface is a mixture of DNI that reaches the earth’s surface without being scattered or absorbed and diffuse horizontal irradiance (DHI), the irradiance resulting from molecules, aerosols, and clouds scattering of the DNI. This partitioning is ever changing because the atmosphere is not static. The governing equation is

GHI DNI sza DHIcos( )= ⋅ + (4.1)

where sza is the solar zenith angle, the angle between the zenith and solar directions. This partitioning is illustrated in Figure 4.1. It is easy to see that the solid dark line GHI is the sum of the direct normal component on the horizontal in gray (the rst term in Equation 4.1) plus the dotted line DHI (the second term of the equation). This is consistent throughout the day until the sun is completely blocked and the rst term on the right-hand side in Equation 4.1 goes to zero, leaving GHI equal to DHI between 16:00 and 17:00 and after 17:30 where the solid dark line and the dotted line coincide.