Results are presented from an experimental programme in the atmosphere to determine water vapour absorption in the 10–12μm atmospheric window. Radiation measurements have been made with upward and downward looking multi-channel radiometers from an aircraft. Midlatitude and tropical conditions have been studied. the wavelength and temperature dependence of the absorption are presented. Possible causes of the absorption are investigated and dimers of the water molecule are concluded to be the responsible absorbing species.