  This study illustrates the application of a random strong line (RSL) model of
radiative transfer to the interpretation of satellite observations of the
upwelling radiation in the 6.3 micron water vapor absorption band. The model,
based upon an assemblage of randomly overlapped, strongly absorbing, pressure
broadened lines, is compared to detailed radiative transfer calculations of the
upper (6.7 micron) tropospheric water vapor radiance and demonstrated to be
accurate to within ~ 1.2 K. Similar levels of accuracy are found when the model
is compared to detailed calculations of the middle (7.3 micron) and lower (8.3
micron) tropospheric water vapor radiance, provided that the emission from the
underlying surface is taken into account. Based upon these results, the RSL
model is used to interpret TOVS-observed water vapor radiances in terms of the
relative humidity averaged over deep layers of the upper, middle, and lower
troposphere. We then present near-global maps of the geographic distribution
and climatological variations of upper, middle and lower tropospheric humidity
from TOVS for the period 1981-1991. These maps clearly depict the role of
large-scale circulation in regulating the location and temporal variation of
tropospheric water vapor.
