The horizontal component of solar radiation

Abstract

By definition, global radiation G is the downward shortwave irradiance, both direct and diffuse, on a horizontal surface. G represents the downwards vertical component of the daylight radiation field and in that conception the horizontal radiation H is then the horizontal component of the daylight radiation field. For a clear sky, H can be expressed as a simple function of direct global radiation, diffuse global radiation and albedo. The characteristics of H differ significantly from those of G. It can be shown that under a clear sky the H climate is more temperate than the G climate all over the world. Some examples are presented where plant morphology and the geographical distribution of plants seem to respond to horizontal rather than to global radiation. However, horizontal radiation sensors are not available and, with occasional exceptions, only global radiation is registered in biometeorological studies. In this paper the construction and calibration of a horizontal radiation sensor are described and results of a field trial with the new instrument are discussed.