A commonly asked question about in-flight icing is how frequently it occurs, at what altitudes and how this varies geographically and by time of the year. A lack of regular, unbiased measurements of the presence and absence of icing conditions makes this very difficult to answer. Thus, other techniques must be used to infer the presence of icing conditions from past observations to create climatologies. In this study, an inferred icing climatology was developed using regularly observed data from 14 years of coincident, 12-hourly US and Canadian surface weather reports and balloon-borne soundings. Although these datasets do not provide direct observations of icing conditions aloft, they have been shown to give strong indications of its presence and absence, when properly combined. The observations are used to infer the characteristics of clouds, including their heights, depths, temperatures, and expected microphysical phase. Cloud base and top height are determined from ceiling observations, and estimated using relative humidity with respect to both water and ice from the sounding, respectively. Likely locations of cloud layers are determined from the relative humidity profile. For each cloud layer, the phase is inferred from a combination of temperature, cloud-top temperature, as well as precipitation and thunder reports from the coincident surface observation using a modified version of the NCAR Integrated Icing Diagnosis Algorithm. The result is an icing "potential" ranging from 0.0 (no icing) to 1.0 (icing extremely likely) for each level in every sounding.

Soundings were examined at 121 sites for 14 years, totaling roughly 10,000 per site and 12 million, overall. Horizontal coverage is fairly uniform across North America, eliminating air-traffic bias from the climatology. Interpolation of results between sites should be reasonable for all but the Intermountain West and, to some extent, the Appalachians and Great Lakes, themselves. Seasonal and monthly results will be presented, as will breakdowns by altitude. Comparisons between PIREPs and output from an independent set of soundings will be shown to demonstrate the validity of the approach. Comparisons will be drawn between results found using this technique and those used in the companion papers (Parts I and III of this series). Using the IIDA sounding technique, icing appeared to be most common along the Pacific Coast, north of the California/Oregon border, with peak frequency in western Alaska. The Great Lakes and eastern Canadian provinces also had a great deal of icing. Icing was at a minimum along and just east of the Rocky Mountains, as well as in the Southwestern U.S. and along the Gulf Coast. Month-by-month results show large deviations from the full-year pattern, as icing prone areas migrate latitudinally with storm tracks, moisture plumes, and of course, common icing temperatures. This is also quite evident in single-station time-height cross-sections.