The spectral distribution of thermal radiation in the earth's atmosphere

• Main Author: Williamson, E. J.
• Published: University of Oxford 1964
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.732594

The humidity of the stratosphere is, from the point of view of radiation transfer studies, a subject of some importance to the meteorologist. It has, nevertheless, presented many problems to the experimentalist. Owing to the extreme dryness of air above the tropopause, compared with that at ground level, great care has to be taken to avoid the contamination of any measuring instrument by water carried up during an ascent into the stratosphere. An indirect method of approach, which to a large extent overcomes these difficulties, is the subject of the work described in this thesis. Measurements of stratospheric humidity which have been made to date by other workers, are reviewed together with some of the theoretical arguments which have been put forward to account for various water vapour distributions. Good evidence, both theoretical- and experimental, exists in favour of a water vapour mixing ratio in the region of 3 × 10^−6 gm/gm in the lower stratosphere. Above about 50,000 ft experimental results have shown considerable diversity, and somewhat larger mixing ratios have been found in many cases. Recent measurements which suggest that the dry region of the stratosphere extends to much greater heights, have thrown doubt on a lot of the early work. The instrument which is the subject of the present study measures the night emission from the 6.3 μ water vapour band in the atmosphere, using a liquid air cooled gold-doped germanium detector. By utilising the variation with height of the boiling point of liquid air, and with it the change in sensitivity of the detector, measurements of emission over about four orders of magnitude have been possible during an ascent. Calibration is performed by means of a black body at a measured temperature. The detector responds to the total incident radiation within a pass band from about 5.5 μ to 7 μ. Since no information is obtained regarding the spectral distribution of radiation within the pass band, but only the combined effect of the whole, a somewhat lengthy analysis has been necessary in order to interpret ascent data in terms of water vapour mixing ratios. Results from one ascent, which was by far the most successful, are discussed in detail. Mixing ratios obtained from the analysis show good agreement with values deduced from radiosonde humidity data below 300 mb and with the well established value a few kilometers above the tropopause. Above this the mixing ratio is found to. vary between 2 × 10^−6 and 4 × 10^−6 up to 25 km. Interpretation of data at greater heights than this presented difficulties due to the presence of a large background component in the measured downward emission. Whilst the exact nature of this emission is not known, it has with some justification been assumed in the analysis that it is not due to water vapour.