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Interpolation of global radiation measurements and estimation of solar radiation

Exploratory research has been carried out to undertake an inventory of radiation balance component data and instrumentation available in Europe, examine the possibility of interpolating global radiation measurements, and formulate practical techniques to estimate solar radiation from existing observations of readily available parameters. Although radiation balance in not usually available over the global telecommunication system (GTS), sunshine hours are often reported. Radiation balance can usually be calculated from sunshine hours using the Angstrom formula. This formula, however, contains 2 empirical constants which are specific to individual locations. The first phase of this exploratory research program included a review of the radiation balance component data and instrumentation available in Europe and resulted in a comprehensive list of stations which collect data on either radiation or sunshine hours. On the basis of the expanded dataset, the 2 empirical constants were estimated on a monthly basis for several hundred European stations. Using these estimations, radiation could be derived at any of the studied stations on a real time basis from sunshine hour values reported over the GTS. To estimate radiation values at other points, it was necessary to examine the seasonal and spatial variation of the 2 constants. It was found that the seasonal variation of the 2 constants fitted various sets of sine and cosine curves. The spatial distribution of the factors was best represented by isolines across Europe. On the basis of these isolines and the trigonometric curves, the values for the constants for any point in Europe could be estimated. So, wherever sunshine hours were reported, radiation could be estimated. Calculation of radiation in this manner was found to be more accurate than direct interpolation of radiation values. In some areas even sunshine hours are not reported, and the research program also developed equations for the estimation of radiation values from cloud cover and daily minimum and maximum temperatures and from daily minimum and maximum temperatures only. The former gives somewhat more reliable results than the latter.

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Joint Research Centre, Ispra
21020 Ispra
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