Spectral Distribution of Solar Radiation in the Nordic Countries

Abstract

In 1977 a cooperative research project between the Nordic countries (Denmark, Finland, Iceland, Norwayand Sweden) was started. The objective was to chart the spectral distribution of solar radiation in the Nordicarea with a view to furnish material to agronomists working on the problem of radiation in relation to plantgrowth and crop yield. Radiation was measured with Eppley spectral pyranometers (PSP) equipped with different glass filters,separate UV detectors and PAR cells. Seven stations participated in the project, giving a longitude span from21 .9°W to 26.7°E and a span in latitude from 55.7 to 69.7°N. This paper presents a preliminary report,giving data for the mean monthly values of the global radiation, the spectral energy distribution and itsvariation between the seven Stations. At two stations (Copenhagen and Aas) measurements were performed throughout the year which madeit possible to investigate the annual variation of the spectral distribution. Results show large annual variations,especially in the ultraviolet and blue regions, which contribute considerably more to the total energy inputduring winter than in summer. The infrared radiation shows an opposite variation with a minimum inwinter and maximum in summer. All of the stations show a marked year-to-year variability in UV radiation,whereas the variation in the other spectral bands are less and, for a couple of stations, almost absent. Duringthe growing season May to September, the spectral distribution of monthly mean varies as follows in percentage of global radiation: In the UV band (XX295-385 nm) it ranges from 5.6% (Troms-, Ultuna) to 4.4%(Sodankyl-); in the blue (XA385-495 nm) the variation is from 19.1% (Copenhagen) to 13.9% (Aas). Thecontribution from the green/orange region (XX495-630 nm) ranges from 21.5% (Reykjavik) to 19.9% (Aas);the red region (AX630-695 nm) from 14.9% (Ass) to 9.4% (Reykjavik), and the infrared region (XX695-2800nm) from 49.2% (Troms-) to 46.2% (Copenhagen). The variation of photosynthetic active radiation (PAR) is ciuite similar to the variation found for thewavelength band XA385-695 nm obtained from the Eppley spectral pyranometers. Evaluating the equivalentwavelength for PAR radiation (explained in text), a long-wavelength shift toward winter is found consistentwith the results from the pyranometers.