Water vapor is probably the most important trace gas in the atmosphere: it is responsible for ~2/3 of the natural greenhouse effect, and plays an important role in numerous physical and chemical processes. Despite its paramount importance, sensitive and reliable water vapor detection systems for automatic airborne operation were not widely used until recently. However, nowadays there exist several projects for monitoring the concentration and the distribution of trace gases (including water vapor) within the atmosphere where there is a strong need for reliable water vapor detector systems.