Atmospheric long wave radiation (Lin) is the most difficult component of the radiation budget to be measured. In Amazonia there are very few regular measurements of this component, even though it is an important variable in the calculation of the surface radiation balance and frequently used in climate models. Given the need for such data, the objective of this study is to evaluate the performance of seven equations used for the estimation of Lin for clear-sky days in forest (Reserva Biológica do Jaru, 10º4'48''S; 61º55'48''W) and pasture (Fazenda Nossa Senhora, 10º45'S, 62º21'W) areas in South West Amazonia. Measurements of atmospheric long wave radiation in the period from June 2005 to May 2006 were compared with estimates. The tested equations performed satisfactorily only during the dry season. High cloud conditions that dominate during the wet season significantly limited the amount of data available to evaluate the equations. Equations that use information about air temperature and vapor pressure for the Lin estimate performed better than those that use only air temperature. The equations of Brutsaert (1975), Idso (1981) and Prata (1996) performed best, had the highest rates of agreement and are therefore the most appropriate equations for estimating atmospheric long wave radiation in South West Amazonia.
Clear-Sky condition; air temperature; water vapor pressure