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Altitude profiles of atmospheric window radiance measured with upward-looking sensors frequently show a rapid decrease in radiance with increasing height over a narrow altitude region in the upper troposphere. This region of rapid decrease is termed a radiometric knee in the altitude profile. The top of this knee defines a radiometric tropopause with a latitudinal height dependence similar to that of the usually defined barometric tropopause. Atmospheric window (10-12-microm) radiance at these altitudes can be associated with the presence of ice particulates. Comparison of the measurements with predicted altitude profiles of atmospheric radiance from the LOWTRAN 7 atmospheric model code shows that a well-defined knee occurs when there is a cloud layer (liquid or ice) such as a subvisual cirrus cloud present. The rate and magnitude of the radiance decrease depend on the optical depth and, therefore, the water content of the layer. Atmospheric background radiance values for near horizontal (large zenith angle) viewing with upward-looking sensors can be as much as a factor of 100 lower above the knee than below it. Comparisons between calculated and observed radiance profiles were used to estimate the vertical extent, total optical depth, and water content of the clouds.

The stratospheric water vapor mixing ratio altitude profile has been derived from spectral observations of the downward night emission from the pure rotation water vapor lines in the 24-29-microm region of the spectrum. The data were obtained during two balloon flights, made on 22 February 1971 and on 29 June 1971, using a balloon-borne spectral radiometer with ~2 cm(-1) resolution. The observed radiances have been fitted to line-by-line, layer-by-layer radiance calculations, from which the water vapor mixing ratio between 10 km and 30 km has been flights show a broad minimum around of 6 x 10(-7)g/g to 4 x 10(-6) g/g.

A grating spectrometer with a Ge: Cu detector was flown on three balloon flights. Spectra in the 4-14.3-micro region were obtained at various altitudes from the ground through 30 km with a resolution considerably better than that achieved in previous flights. Some of the spectra were obtained over long paths at float altitude, during the sunset. Data from these flights are presented with a discussion of the significant features of the observed absorptions. Special emphasis is put on the new features observed during the sunset.

A balloon-borne spectrometer was used to measure the atmospheric absorption of solar radiation in the 9-10-micro region as a function of altitude. Among the numerous spectra recorded, several were made with very long optical paths above the troposphere obtained from floating altitude (~30 km) while the sun set. When the experimental results are compared with calculated spectra, based on line by line parameters of the nu(3) and nu(1) fundamentals of ordinary ozone, (16)O(3), combined with the Curtis-Godson approximation, large discrepancies are found for the long path spectra. It is shown that the hot bands nu(3), + nu(2), - nu(2), and nu(3), + nu(3), - nu(3), of the (16)O(3), and the nu(3) fundamentals of the isotopic species (16)O(18)O(16)O and (16)O(16)O(18)O, can contribute significant absorption for long paths such as obtained in this experiment. Such long paths also give rise to significant absorptions by the nu(3) - nu(1) and the nu(3) - 2nu(2) degrees bands of CO(2). Including these weak bands in the calculated spectra leads to good agreement with the observed data.

A balloon-borne, 0.5-m, Czerny-Turner grating spectrometer has been designed and constructed at the University of Denver and has measured atmospheric transmittance in several spectral regions between 2 micro and 14 micro. These data have been obtained on a series of eleven balloon flights at three geographic locations over a period of thirty months. The solar pointing system, spectrometer optics, electronics and recording system are described. The preflight and flight performance of the spectrometer is discussed and sample data presented.