Measurement of Surface Radiation

Total short wave 'solar' radiant flux density on a horizontal surface on the earth's surface (BOA), that is, global radiation, Eg#, is measured with a pyran-ometer. First class pyranometers measure the temperature difference between an exposed optically black surface and either a white surface (in the older instruments) or the lower non-exposed surface using a thermopile. In order to exclude thermal radiation and advection of heat from the surroundings the black surface is covered with two quartz glass domes which transmit radiation between 200 and 4500 nm wavelength, and a temperature correction circuit is incorporated into the instrument. Another type of 'pyranometer' in common use, due to its lower cost, is based on a selenium cell which upon illumination causes an electrical current to flow. The sensor is covered with appropriate filters to measure solar radiation, but the maximum wavelength measured is 1100 nm, so total solar radiation is determined indirectly by assuming that the ratio of the full spectrum to that below 1100 nm is constant. In most outdoor conditions the assumption is good enough for many applications, for example, calculation of crop water requirements, but the non-thermopile pyranometers are not acceptable for first class meteorological measurement.

Frequent cleaning of the dome and yearly calibration of sensors is necessary in order to ensure the reliability of measurements. These and other constraints have led to sparse measurement networks producing reliable data for solar radiation as compared to those measuring air temperature. Most of the networks began to operate during the International Geophysical Year, 1957 1958.

A second widely used surface measure which has been of interest is sunshine duration (SSD), or the amount of time that direct solar radiation exceeds a threshold of 120 Wm 2, corresponding approximately to direct irradiance at 3° solar elevation under clear sky conditions [1]. This measure has been shown to be highly correlated with global radiation, both on a single day basis as well as for yearly totals [18,19]. Instruments measuring SSD came into use in the nineteenth century, and some of their history has been recently reviewed [20]. Many measurement series dating back to the nineteenth century are available in various forms, and analysis of these has enabled a rough view of variations in solar radiation for more than a century (e.g. [21,22]).

In addition to surface measurements, satellite based sensors have been monitoring earth radiance in different wavebands for more than two decades. Algorithms have been developed to use these measurements to calculate solar radiation at the surface. These measurements have the advantage of spatial averaging over an area several orders of magnitude larger than the few square centimetres measured by the surface based sensors, and the ongoing efforts to improve the reliability and accuracy of the satellite measurements has led to their increased acceptance.

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