Effects of clouds on erythemal and total irradiance as derived from data of the Argentine Network

Abstract

Ultraviolet (UV) erythemal and total (300–3000 nm) irradiance measurements of the Argentine Servicio Meteorológico Nacional Network were related to ground‐based cloud observations. No geographical dependence was observed in the effects of each cloud‐type on the irradiance, from tropical to Antarctic regions. For overcast conditions, median transmittance percentages with respect to the clearsky situation of 81%, 44% and 36% at high, medium and low clouds respectively for erythemal irradiance, and 83%, 30% and 23% for total irradiance were determined, similar to results at mid‐latitudes of the Northern Hemisphere. Irradiance enhancement by broken cloud fields is more pronounced from 5 to 7 octas cloud coverage and can last even hours, with peak instantaneous values of 113% for erythemal and 133% for total irradiance, with respect to the very clean clearsky situation. In each case, the total irradiance is usually more attenuated and also more enhanced by clouds than the erythemal irradiance.