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Observations in the spectral window at 11–12 μm, in the carbon dioxide band at 13.3–13.4 μm, and in the water vapor rotation band at 18.7–18.8 μm have been made by the satellite infrared spectrometer and the vertical temperature profile radiometer on the Nimbus 4 and NOAA-2 satellites. Combinations of radiances measured by the separate instruments have been used to infer sea-surface temperatures and mesoscale precipitable water in tropical regions. In each spectral interval, atmospheric transmittances for these analyses were constructed from continuous absorption (of unexplained origin), which is dependent upon the square of the partial pressure of water vapor and about the inverse fifth power of temperature, in addition to absorption by resonance lines. Results are consonant with those from other laboratory and atmospheric transmittance measurements, and they underline the importance of properly modeling the water-vapor continuum in calculations of atmospheric radiation in the middle infrared.

The Satellite Infrared Spectrometer (SIRS) on the Nimbus III satellite was designed to measure the earth’s spectral radiances in the 15-μm band of carbon dioxide. From simultaneous measurements of spectral radiances it is possible to obtain the vertical temperature profile of the atmosphere. The measurements are approximated by the integral equation of radiative transfer, modified by one or two layers of clouds. A solution requires that the surface radiative temperature and the surface air temperature be known. By iteration, a solution based upon the statistical behavior of the atmosphere is obtained for the free air temperature and the cloud heights and amounts. Examples are presented, comparing the SIRS soundings with coincident radiosonde soundings. The results from this experiment indicate that the technique can be applied as a routine observing tool for meteorological use.

The meteorological satellite TIROS II carries a five-channel radiometer which scans the earth by rotation of the satellite. Two channels are sensitive to sunlight reflected from the earth and three are responsive to terrestrial infrared emission. The effect of the optical properties upon the measurements is indicated. Calculations, using model atmospheres, show the sources of outgoing terrestrial radiation and limb-darkening effects for two of the channels. A map of the radiation received by the channel sensitive in the window region (8–12 µ) is compared with a conventional weather chart.

The infrared spectral radiance has been calculated near the limb of the earth, using a curved refracting atmosphere. Radiance was calculated in narrow spectral intervals for four atmospheric models representing a wide range of meteorological conditions. Comparison is made of the radiance across the limb for each model atmosphere in several narrow spectral intervals; two hypothetical filters are then convoluted with spectral radiance to show the effective radiance across the limb, as it might be viewed by a horizon sensor. The results indicate that the 15-μ carbon dioxide band and the rotational vapor band are best suited for horizon sensing and that neither is decisively superior on the basis of radiance. It is also shown that radiative flux calculated using the exact radiance values near the limb is only about 1% more precise than that calculated from an extrapolation to the limb of radiance values determined at zenith angles where the plane parallel approximation is valid.

The detailed spectral absorptances by atmospheric slant paths in the oxygen “A” band have been calculated, using parameters deduced from very high- and medium-resolution spectra and from available data. Special attention is given to the potential use of satellite measurements of sunlight reflected from clouds to determine their heights. A complete table of wavelengths and other line parameters has been prepared, based largely upon the work of Babcock and Herzberg; lines of the O^16O^18, O^16O^17, and O_2^16 have been included. From the equivalent widths of several lines and from an iterative calculation and comparison procedure, it was found that the Einstein coefficient for the band transition is Amn = 0.145 sec^−1. From a standard atmosphere the absorptance in two selected intervals as functions of height (or pressure level) of the reflecting surface and of the slant angle have been calculated, employing point-by-point calculation and quadrature. It is found that over most of the atmosphere cloud heights can be determined to better than 1 km if the errors of measurement do not exceed 4%. The dependence of the absorptance on the vertical temperature profile is shown to be small.

The calibration of the television cameras to be carried in the Nimbus meteorological satellites requires that the relative spectral responses of the vidicon systems be known. In the method developed for this purpose, a series of carefully selected optical filters was interposed between a known constant source and the camera lenses. From the measured outputs of the vidicons, the relative spectral responses were derived as the solution of an integral equation of the first kind. This solution, however, is generally highly unstable, but a physically acceptable solution may be obtained by imposing an appropriate smoothing condition on it. This technique was tested successfully with measurements by a brightness meter, and with simulated outputs by Tiros vidicons of known spectral responses. The spectral responses of two Nimbus vidicons, derived from measurements, are shown.