Solar and Infrared Radiation Measurements

4 Direct Normal Irradiance

We receive from the sun perfectly enormous quantities of radiation.

Charles Greeley Abbot

(1872–1973)

4.1 Overview of Direct Normal Irradiance

Solar radiation that arrives at the earth’s surface having come directly from the sun is defined 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 ...