Fast-response tunable diode laser measurements of water vapor were made over the North Atlantic during the Subsonic Assessment Ozone and Nitrogen Oxide Experiment (SONEX) conducted in the fall of 1997. Resulting water vapor mixing ratios, in conjunction with air temperature and pressure measurements obtained by the DC-8's Data Acquisition and Distribution System (DADS), were used to calculate the prevalence of ice-saturated conditions (relative humidity with respect to ice>100% or frost saturation) in the upper troposphere over the SONEX sampling region. Additionally, Appleman theory was applied to the data to determine the subset of ice-saturated regions which would support formation of contrails. Results suggest that studies to determine the potential climate or radiative impact of contrails and aviation-induced cirrus clouds should not only consider the climatology of frost-saturated regions, but should also address whether ambient conditions can support contrail formation within these regions. A separate discussion within the paper describes an in-flight intercomparison between the SONEX diode laser hygrometer and the Pollution From Aircraft Emissions in the North Atlantic Flight Corridor (POLINAT 2) cryogenic hygrometer and shows that the instruments perform to within their stated accuracies (~10%) over a range of mixing ratios and altitudes. ¿ 2000 American Geophysical Union