solar constant

The solar constant is a fixed quantity for the solar radiation outside the atmosphere. It refers to the solar irradiance of one square meter and was set by the World Meteorological Organization at 1.367 W/m2 and is approximately constant in space.

On the Earth's surface, we speak of irradiance, rather than solar constant, given in watts per square meter, and of global radiation. This is the sum of the solar radiation in Wh/m2 related to one year .

Global radiation is less than the solar constant because it is affected by the atmosphere, the angle of incidence which depends on the day and season, the latitude, as well as reflections, absorptions and scattering.

Air Mass (AM) at an angle of incidence of 90° and 41.8°.

Therefore, the irradiance values vary on the Earth's surface and range from a maximum of 800 kWh/m2 to 1,200 kWh/m2. The reduction of solar radiation is expressed by the Air Mass( AM). This factor for the thickness of the atmosphere varies in our latitudes depending on the season and is 1.5 in summer when the sun is high and 4 to 15 in winter when the sun is low.