Use and Misuse of Water Vapor Imagery

Water vapor satellite images are frequently misused and misinterpreted.  Here's a very basic primer.

Water vapor images do not measure water vapor and they certainty do not measure water vapor in the lower atmosphere where it plays an important role in the development of precipitation systems.  Water vapor imagery is simply based on a band of infrared radiation that is absorbed and emitted by water vapor.  This contrasts with the band of infrared radiation sampled by conventional infrared satellite images which is largely unaffected by the presence of water vapor or other atmospheric gases.

As a result, water vapor imagery is strongly influenced by the distribution, temperature, and concentration of clouds and water vapor in the upper troposphere, typically at altitudes above 500 mb (5500 meters).  A prime example is the water vapor loop below, to which I've added contours of precipitable water (i.e., total integrated water vapor).  Note the northward spread of cirrus clouds and upper-level moisture from into Utah over the past two days.  As ominous as this looks, it is simply a very thin, cold layer of clouds and moisture.  The juicy air associated with tropical storm Simon remains well to the south, as indicated by the precipitable water contours, and is just beginning to push across the US–Mexico border.  

Even areas in red above, which many people interpret as "dry" can overlay areas of abundant low-level moisture.  Those are areas that are dry and warm in the upper atmosphere, but not necessarily in the lower atmosphere.  For example, in the wake of tropical storm Simon, the upper-levels are very dry and warm, but the precipitable water values are fairly high due to abundant low-level moisture.

One of the major advantages of water vapor imagery is the ability to track water vapor features in areas that are cloud free.  This is extremely useful for inferring upper-level winds when no clouds are present.  One can also identify smaller-scale upper level features such as short-wave troughs using water vapor imagery.  These are effective applications of water vapor imagery.  The use of water vapor imagery to infer low-level water vapor concentrations is, however, dubious and should be avoided.