The global distribution of water vapor in the middle atmosphere of Venus

The mean, solar-fixed horizontal and vertical distribution of water vapor in and above the Venusian cloud layer is presented. This is derived from far-infrared measurements made by the Orbiter Infrared Radiometer (OIR) instrument of the Pioneer Venus mission in the rotation band of water vapor at 45 μm, and from the mean solar-fixed temperature field and cloud structure retrieved from temperature soundings by the same instrument in five spectral channels. The water vapor retrieval scheme is discussed together with the calculation of water vapor transmission functions and their experimental verification. The sensitivity of the results to measurement errors and cloud microphysical properties is also considered. Mean water vapor column abundances above cloud unit optical depth at 11.5 μm are found to be greatest at equatorial latitudes in the early afternoon, reaching 50 ± 20 precipitable microns (100 ppm), and fall to less than 3 ± 2 precipitable microns (6 ppm) on the nightside of the planet. On the nightside mixing ratios fall monotonically with altitude, whereas dayside mixing ratios frequently increase with altitude near cloud unit optical depth. These results are broadly consistent with those of earlier Earth-based measurements.