A climatology of icing conditions would provide a better understanding of the phenomena that result in aircraft icing. As discussed in part I of this series, the only direct observations of icing events are pilots' reports (PIREPs), which are non-systematic and spatially biased. Fortunately, icing forecasts and diagnoses can also provide information on climatological characteristics of aircraft icing. In this paper, the official Aviation Weather Center (AWC) forecasts of icing events, i.e. icing AIRMETs, and the Integrated Icing Diagnostic Algorithm (IIDA), are used to assess the climatological characteristics of aircraft icing.

Icing AIRMETs for the coterminous United States issued during the winters of 1994-2000 are evaluated. IIDA is only available for two winter seasons, 1999-2000 and 2000-2001. Contour maps of the relative frequency of both information sources are presented for the winter season in addition to individual months. Icing conditions vary by altitude, so climatological characteristics in certain altitude ranges are also considered. Finally, these climatologies are compared and contrasted to those presented in parts I and II of this series.

None of these sources directly and systematically measure aircraft icing, so they are not ideal. However, taken together they can paint a reasonably good picture of the climatological nature of aircraft icing conditions.