AbstractThe use of emissivity for calculating atmospheric radiation fluxes due to water vapour is investigated with a view to finding the most accurate way of using the approximation. It is shown that with care it is possible to adjust the emissivity so that errors in fluxes of less than 0·05 mW cm−2are incurred, when compared with calculations using more accurate methods. This can be attained with the aid of a least squares fit of emissivity on calculated flux. ‘Laboratory’ type emissivities give errors of up to 1·3