Most solar cells receive radiation not only directly from the sun disc (direct radiation), but also from a large portion
of the sky (diffuse radiation). Hence, a solar spectrum needs to include the radiation coming from the sky to be relevant to PV.
This schematic suggests that the reflection of sunlight at the ground influences the appearance of the sky. For example, if the
ground is covered by snow, sunlight is strongly reflected back to the sky, where it gets scattered again in all directions – so,
to an observer on the ground, the blue sky appears approximately double as bright as without snow. This effect is called backscattering.
The amount of backscattering depends on the broadband average of the reflectance on the ground, called albedo.
The figure on the left shows typical values .
The albedo is usually measured from perpendicular above the ground as the ratio of reflected (upwelling) irradiance to
incident (downwelling) irradiance. It is generally accepted only for horizontal surfaces.
Note in the figure on the left that many types of vegetation have an albedo near 20%. Therefore, this is an often used
approximative value . The reflection is often very diffuse, so the variation over a large part of the day is rather small ,
as is shown in the figure on the left. In contrast, the albedo over a water surface depends on the angle of the sun over the horizon .
Figure: Albedo of various environments (left) and Albedo vs the sun's elevation (right).
(The left figure contains Google Map images of Australia and Switzerland.)