Direct Method of Measuring Stratospheric Water Vapour Mixing Ratios

THE measurement of the mixing ratio, μ, of water vapour to air in the stratosphere has been carried out many times, using spectroscopic methods, from both aircraft 1,2 and balloons 3–6 . As a result of the range of values that have been obtained the subject has become controversial 6,7 . We present here some submillimetre wavelength measurements made in an aircraft observing atmospheric emission, using methods of Fourier transform spectrometry 8 , which may provide an improved method of obtaining this parameter. Previous sub-millimetre atmospheric emission measurements 9 have produced estimates of stratospheric water content, but the quantitative value of these results is limited because of the problem of radiometrically calibrating the system in order to obtain reliable measures of background and zero levels. In the present work the direct comparison of water vapour and oxygen emission has largely removed these requirements and greatly simplified the analysis.