The radiation reaching the earth's surface can be represented in a number of different ways. Global Horizontal Irradiance (GHI) is the total amount of shortwave radiation received from above by a surface horizontal to the ground. This value is of particular interest to photovoltaic installations and includes both Direct Normal Irradiance (DNI) and Diffuse Horizontal Irradiance (DIF).

DNI is solar radiation that comes in a straight line from the direction of the sun at its current position in the sky. DIF is solar radiation that does not arrive on a direct path from the sun, but has been scattered by molecules and particles in the atmosphere and comes equally from all directions.

On a clear day, most of the solar radiation received by a horizontal surface will be DNI, while on a cloudy day most will be DIF.

If you are interested in more detailed information on the GHI values for any location worldwide, please see our Solar Prospecting Tools, Solar GHI GIS Data Layer, 3TIER Solar Time-series, and FullView Solar Site Climate Variability Analysis products.