THE HORIZONTAL COMPONENT OF SOLAR-RADIATION

By definition, global radiation G is the downward shortwave irradiance , both direct and diffuse, on a horizontal surface. G represents the d ownwards vertical component of the daylight radiation field and in tha t conception the horizontal radiation H is then the horizontal compone nt of the daylight radiation field. For a clear sky, H can be expresse d as a simple function of direct global radiation, diffuse global radi ation and albedo. The characteristics of H differ significantly from t hose of G. It can be shown that under a clear sky the H climate is mor e temperate than the G climate all over the world. Some examples are p resented where plant morphology and the geographical distribution of p lants seem to respond to horizontal rather than to global radiation. H owever, horizontal radiation sensors are not available and, with occas ional exceptions, only global radiation is registered in biometeorolog ical studies. In this paper the construction and calibration of a hori zontal radiation sensor are described and results of a field trial wit h the new instrument are discussed.