Direct and diffuse radiation
Radiation & sun beam and diffuse radiation
Direct radiation arrives from the solar disc. Diffuse radiation has been scattered in the atmosphere and reaches us from many parts of the sky.
Look at the shadow
Under a clear sky, the sun casts a sharp shadow because direct light makes up a large share. Under thick cloud, those shadows often disappear even though it is still light. Most remaining light is diffuse, arriving from many directions.
What the atmosphere does to light
Air molecules, aerosols and cloud droplets redirect some sunlight. Scattered light also reaches places without a clear view of the sun. Other radiation is absorbed or reflected back into space.
Which values can be added?
On a horizontal surface, direct and diffuse components add up to global horizontal irradiance, measured in W/m². The values must describe the same surface and time period.
DNI, however, means Direct Normal Irradiance. “Normal” means perpendicular here: the measuring surface faces the rays squarely. This is not the direct component falling on a horizontal surface. The solar angle must be accounted for before adding it.
Shade still receives radiation
Diffuse light explains why solar panels can produce electricity under overcast skies. That does not mean they produce as much as in sunshine. Equally, the absence of a sharp shadow does not automatically remove the need for UV protection.