Values of the Angstrom's turbidity coefficient, beta, 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 beta at Nakhon Pathom (13.81 degreesN and 100.4 degreesE) using narrow-band spectral irradiance data obtained from a multi-filter rotating shadow band radiometer. Langley's method was employed to calculate 0 from the spectral data. The values of 0 derived from this method were used as references to validate a method for computing beta 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 beta at meteorological stations situated at four main cities, namely Chiang Mai (18.78 degreesN, 98.98 degreesE) located in the north, Ubon Rachathani (15.25 degreesN, 104.87 degreesE) in the northeast, Songkhla (7.20 degreesN, 100.60 degreesE) in the south and Bangkok in the central region. Based on values of beta of these cities, a new model relating P to visibility, suitable for the tropical climate was developed. This model was used to estimate beta at the other 48 meteorological stations where the visibility was routinely observed. Finally, seasonal variations of beta were investigated. It was found that for the stations in the north, the northeast and the central region, the values of beta are relatively high in the dry season (November-April). They decrease in the wet season (May-October). For most stations in the south, beta 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 beta. (C) 2003 Elsevier Science Ltd. All rights reserved.