solarirradiation

Solar irradiation, or solar radiation, is the radiant energy from the Sun received per unit area at a surface. It is a fundamental input for climate studies, weather, and solar energy technologies, and it encompasses the energy that reaches the Earth’s surface as well as the portion intercepted by the atmosphere.

At the top of the atmosphere, the solar constant is about 1361 watts per square meter (W/m^2). After passing through the atmosphere, the energy reaching the surface varies with the sun’s angle, atmospheric composition, and surface properties. In solar engineering, three principal components are commonly used: global horizontal irradiation (GHI), direct normal irradiation (DNI), and diffuse horizontal irradiation (DHI). The observed GHI is the sum of the direct irradiance arriving at a surface perpendicular to the sun’s rays (DNI) projected onto the horizontal, plus the diffuse irradiance scattered by the atmosphere.

Atmospheric scattering and absorption by gases, water vapor, and aerosols reduce irradiance. Rayleigh scattering favors shorter wavelengths, while aerosols influence both attenuation and spectral quality. Cloud cover can cause rapid and large fluctuations in irradiance. The diurnal cycle, seasonal tilt of the Earth, and latitude determine the magnitude and timing of solar irradiation at a given location.

Measurement of solar irradiation uses instruments such as pyranometers (for GHI and DHI) and pyrheliometers (for DNI). Data support solar energy system design, building energy modelling, and climate research. Under standard test conditions for photovoltaic testing, the spectrum is often described as AM1.5G with about 1000 W/m^2 at solar noon in mid-latitudes.

Applications include sizing and performance estimation for photovoltaic and solar thermal systems, daylighting analysis, and solar resource mapping for planning and policy.