Global radiation is the total amount of solar radiation that hits a horizontal surface on the ground. It's made up of two components: direct radiation and diffuse radiation. Direct radiation comes directly from the sun, while diffuse radiation is scattered by atmospheric particles like clouds and dust.
Global radiation can be used as a standard to compare sites regardless of their latitude or solar position. It can also be used to characterize power generation, which depends on global radiation, ambient temperature, and other factors.
Global radiation is stronger at midday than in the morning or evening, and in the summer than in the winter. It's also stronger the closer you are to the equator. For example, Germany has an annual value of 900–1200 kWh/m2 of global radiation, while Spain has around 2000 kWh/m2.
Atmospheric conditions can reduce direct beam radiation by up to 100% on cloudy days. This is because the atmosphere modifies the spectrum of radiation through absorption and scattering. For example, atomic and molecular oxygen and nitrogen absorb very short wave radiation, effectively blocking radiation with wavelengths less than 190 nm.
Global radiation can be measured with a pyranometer.
