This paper presents a technique for mapping monthly average hourly downward longwave (LW down arrow) irradiance using ground- and satellite -based data. A model relating LW down arrow irradiance to a satellite derived brightness temperature of the earth -atmospheric system, relative humidity and ambient air temperature was formulated. This model was validated against LW down arrow irradiance obtained from measurements at 4 sites in the tropics and discrepancy in terms of root mean square errors and mean bias errors was found to be 1.89% and -0.69%, respectively. After the validation, the model was used to calculate monthly average hourly LW down arrow irradiance over Thailand and the results were displayed as LW down arrow maps. These maps reveal the influence of various factors on LW down arrow irradiance. (C) 2016 Elsevier Ltd. All rights reserved.