Spectral Distribution of Solar Radiation on Clear Days: A Comparison Between Measurements and Model Estimates

Abstract

The spectral distribution of the direct solar and scattered sky radiation are computed and presented as functions of solar height and as integrated daily values for selected days of the year. The monochromatic fluxes are integrated over the wavelength range covered by the Schott glass filters GG14, RG2 and RG8 as well as the ultraviolet filter used by Eppley Laboratories in their precision spectral pyranometers, in order to compare the theoretical results with values measured by these instruments. In the model, the atmosphere is considered to be plane parallel with infinite lateral extent and free from aerosols and clouds. With reasonable assumptions about the atmospheric content of ozone and water vapor, the ultraviolet (λ = 295–385 nm) part of the daily integrated global radiation range from 4 (winter) to 5% (summer) at 59.7°N latitude (Agricultural University, Aas). The corresponding values for the blue (λ = 385–495 nm), green/orange (λ = 495–630 nm), red (λ = 630–695 nm) and infrared (&lambda = 695–2800 nm) parts of the spectrum are, respectively, 14–16, 16–19, 8(±0.5) and 58–51%. As a general result, the model predicts the spectral distribution of global radiation to be almost insensitive to solar height above ∼15°. Above this height (for summer conditions), approximately 5.5% of the global radiation consists of ultraviolet radiation, 16% of radiation falling in the blue wavelength band, 19% in the green/orange, 8% in the red and 51.5% in the infrared band. With snow-covered ground the above numbers are modified by a few tenths of a percent with the greatest change in the ultraviolet spectral band. The model's prediction of a nearly constant spectral distribution when the solar height is greater than ∼15° is not supported by measurements, which on the contrary show a marked blue shift in global radiation toward solar noon. The solar height effect causes a considerable annual change in the spectral distribution of the daily global energy flux. For the month of March, 4.2% of the energy is due to ultraviolet radiation, 8.9% to radiation in the blue spectral band, 21.8% in the green/orange band, 12.4% in the red and 52.9% in the infrared. For July, the corresponding figures are 4.6, 16.1, 18.0, 10.7 and 50.6%. Compared to the model estimates given earlier, the model is seen to give reasonable results for the spectral distribution of global radiation on a daily basis, although the changes in spectral distribution throughout the day are not accounted for.