B.2.5—Solar constant S

Solar constant

The solar constant, $$S$$, is the solar-radiation intensity received per unit area at the Earth’s orbital distance, with the surface perpendicular to the incoming rays. Its units are W m⁻².

It is not the global mean

The solar constant describes the incident intensity on a surface facing the Sun. A planet’s spherical geometry spreads the intercepted power over a larger total surface, so the planet-wide mean incoming intensity is smaller.

Use it as an input

In an energy-balance problem, S is the incoming solar intensity before accounting for the planet’s projected area, albedo, atmospheric absorption or averaging over the whole sphere.

Common trap

Do not call the solar constant the total solar power intercepted by Earth. It is an intensity: power per unit area.