All published work on the water-vapour continuum in the region 8 μ to 50 μ is reviewed in the light of some new experimental results in the 11 to 21 -μ region. This new work employed a 15.5 m multiple-reflection cell, giving total path lengths up to 500 m, and a high-resolution (1-2 cm^-1) grating spectrograph. Control of temperature (20-40°C), and water-vapour partial pressure (3-35 mb) allowed simulation of tropospheric conditions and their extension to rather higher humidities, but the total pressure could not be varied from atmospheric.

The results suggest that two mechanisms of absorption are involved. One of them is almost certainly the foreign-broadening of water-vapour lines and it is responsible for most of the continuum absorption noted by previous workers. The origin of the other is not clear; it gives rise to absorption which increases with partial water-vapour pressure, e, equalling the foreign-broadened component at e 15 mb. This behaviour was first reported by Bignell, Saiedy and Sheppard (1963) who then attributed it to self-broadening of H₂O lines. However, the present more sensitive work casts doubt on this interpretation because the absorption is found to have a negative temperature dependence of 2 per cent per °C over the range 21° to 45°C, which is greater than that of any water-vapour line. Absorption of the same type, though weaker, is present near 4 μ.

Reinterpretation of previous open-air measurements in atmospheric conditions ranging from warm and damp to cold and dry shows that the systematic differences they exhibit might be a manifestation of the new absorption.

No firm explanation can be given, but the possibility of continuum absorption by the recently reported water dimer molecule (H₂O)₂ should be investigated.