Extraterrestrial solar radiation
Radiation & sun Top-of-atmosphere solar radiation TOA irradiance extraterrestrial irradiance
Extraterrestrial radiation is solar radiation at the top of Earth’s atmosphere. It describes the available sunlight before clouds and air modify it.
Extraterrestrial, without the visitors
Here, “extraterrestrial” simply means outside Earth’s atmosphere. It describes sunlight before it passes through the air. This is a calculated reference value, not a measurement from the roof.
What our display means
The value refers to an imaginary horizontal surface at the top of the atmosphere. It depends on solar position, latitude, season and the changing Earth–Sun distance. The horizontal value is zero at night.
Hourly pro-page values are in W/m², averaged over the preceding hour. Clouds and air pollution do not alter this reference calculation: their effects occur below it.
Comparing three radiation values
- Extraterrestrial: before sunlight travels through the atmosphere.
- Clear-sky surface radiation: calculated without clouds, but with an atmosphere.
- Global horizontal radiation at the surface: light arriving under observed or modelled conditions.
Why compare them?
The values help assess the atmosphere’s influence. Their difference alone cannot identify whether cloud, water vapour or aerosols had the greatest effect. Averaging periods and surface orientations must also match.