It is a direct experience, although some intuitive explanation, the sun heats less the further from the zenith, either during the day or through the seasons. Among the various causes, the most prevalent is the height of the Sun above the horizon or equivalently, the value of the angle between the distance to this star segment and the ray pointing to the zenith. This variation produces that the same amount of radiation of energy emitted covers on different amount of surface.
Two ways of looking at the phenomenon: