Angle of Incidence

the angle that a ray (of solar energy, for example) makes with a lineperpendicular to the surface. For example,a surface that directly faces the sun has a solar angle of incidence of zero, but ifthe surface is parallel to the sun (for example, sunrise striking a horizontal rooftop),the angle of incidence is 90&#176. The figure accompanying the description ofairmass illustrates a solar angle of incidence of48.2&#176 to a horizontal surface.