Simplified Techniques to Study Components of Solar Radiation Under Haze and Clouds

• Published in: Journal of applied meteorology (1962) Vol. 21; no. 3; pp. 373 - 383
• Main Author: Wesely, Marvin L.
• Format: Journal Article
• Language: English
• Published: American Meteorological Society 01-03-1982
• Subjects: Analytical Methods and Instrumentation; Clouds; Cosine function; Diffuse radiation; Photosynthetically active radiation; Pyranometers; Radiometers; Sensors; Silicon; Solar radiation; Zenith
• ISSN: 0021-8952; 2163-534X
• DOI: 10.1175/1520-0450(1982)021<0373:sttsco>2.0.co;2

Estimates of the global (G), diffuse (D) and direct-beam (I) irradiances at the surface of the earth can be obtained with a single instrument, the "dial" radiometer. The dial assembly intermittently shades a solid-state sensor on a continual automatic basis. This is a very simple instrument that does not require mechanical adjustments of the shade. When corrections for imperfect cosine response and excessive shading of sky radiation are performed, measurements averaged over 1 h should be accurate well within ±5%. Estimates of atmospheric turbidity or haziness can be expressed as an extinction coefficient, computed for I in reference to that obtained under cloudless clean skies for the same solar zenith angle. The uneven spectral response of silicon-cell and PAR (photosynthetically active radiation) sensors should be considered when comparing estimates of G, D or I to measurements of these components by a wide-band sensor. Linear relationships seem adequate for a variety of cloud conditions. This allows the use of a single dial silicon-cell radiometer, for example, to estimate quite accurately the values of G, D and I that would be seen by wide-band or PAR radiometers. An alternative, but less exact, means of obtaining estimates of hourly averages of D and I is to measure only G and use the ratio of G to that which would be obtained under clean, cloudless conditions as the sole determining factor.