Determination of Angstrom’s turbidity coefficient over Thailand

Values of the Angstrom’s turbidity coefficient, β, at 53 meteorological stations covering Thailand were determined by using three different methods. A selection of the methods was based on input data available at each station. It was started with the calculation of β at Nakhon Pathom (13.81 °N and 100.4 °E) using narrow-band spectral irradiance data obtained from a multi-filter rotating shadow band radiometer. Langley’s method was employed to calculate β from the spectral data. The values of β derived from this method were used as references to validate a method for computing β from broad-band direct irradiance proposed by Louche et al. (Solar Energy 38(2)89). It was found that this method was valid for a tropical climate. Then Louche et al.’s method was used to calculate β at meteorological stations situated at four main cities, namely Chiang Mai (18.78 °N, 98.98 °E) located in the north, Ubon Rachathani (15.25 °N, 104.87 °E) in the northeast, Songkhla (7.20 °N, 100.60 °E) in the south and Bangkok in the central region. Based on values of β of these cities, a new model relating β to visibility, suitable for the tropical climate was developed. This model was used to estimate β at the other 48 meteorological stations where the visibility was routinely observed. Finally, seasonal variations of β were investigated. It was found that for the stations in the north, the northeast and the central region, the values of β are relatively high in the dry season (November–April). They decrease in the wet season (May–October). For most stations in the south, β was relatively low and remained nearly constant all year round. It was also inferred that the northeast monsoon and the southwest monsoon had a strong influence on the seasonal variations of β.