For many years sunshine hours have been recorded using manually operated meteorological stations. This measurement was originally developed as a means of recording integrated global solar radiation, before the advent of electronics. By using modern instruments and datalogging systems, there are now often more appropriate ways of recording solar radiation for scientific applications. However, because there is an archive of historical sunshine data, and also because sunshine hours are now commonly used as a measure which the public can relate to (normally in weather forecasts and tourist brochures), there is a requirement to continue making these measurements.
Traditionally this has been done using a Campbell-Stokes recorder, which consists of a glass sphere with a piece of card held at a defined distance behind the glass. The card is graduated with time intervals which match the movement of the image of the slln as it tracks across the sky. If the sun is strong enough then the focused beam carbonises the card leaving a trace. The length of the trace equates to sunshine duration. However, reading the cards always involves a degree of subjectivity and the design does not lend itself to automation.
A formal definition of sunshine duration by the World Meteorological Organisation (WMO) has allowed the development of automatic instruments for measuring sunshine hours. This defines sunshine hours as 'the sum of the time intervals (in hours) during which the direct (normal) solar irradiance exceeds a threshold of 120Wm-2 . The most complex (and most accurate) of these instruments are tracking pyrheliometers, where a collimated sensor automatically moves to track the movement of the sun. This reads the direct beam radiation only. Any reading over 120Wm-2 is defined as being sunshine. However, as this type of sensor does not truly track the sun it requires regular adjustment to take into account the seasonal changes in solar declination. Also, this sensor is relatively expensive and has moving parts requiring extra power. A much simpIer approach is to try to estimate sunshine hours from the single pyranometer/solarimeter of the type normally installed on most weather stations. As these sensors measure total global radiation the normal definition of 'sunshine' cannot be used. Simple fixed thresholds, as often used in low grade weather stations, do not give reliable answers either, as diffuse radiation from a completely cloudy sky in the summer wiJl often exceed direct beam radiation in the winter. An alternative algorithm has been sllggested by workers at the Royal Dutch Meteorological Institute (KNMI). They have proposed and tested an algorithm (Slob-algorithm) which defines sunshine as being when the measured global radiation (G) is greater than 0.4 times the potential solar radiation outside the earth's atmosphere on a horizontal surface (Go). One long term test of this algorithm showed that estimates of sunshine hours were on average within 0.9 hours of the daily total (Slob, 1991). While this might appear to give rather poor accuracy compared to that one would expect for totalised solar radiation, they consider it accurate enough for normal non-scientific use of sunshine hour data. As the sensor can also be used to make accurate solar radiation measurements, scientific data can be collected at the same time. Further development resulted in an alteration of this algorithm in 1993 (Algorithm Bergman). In this study the results for "sunshine-duration" calculated by both algorithms will be compared to measurements from the Campbell-Stokes. The results of the comparison will be discussed and concluding remarks wil1 be given.
Jarno Schipper. Vergelijking van diverse methodes voor de berekening van zonneschijnduur uit globale straling
KNMI number: TR-268, Year: 2004, Pages: 44