A climatology of high-altitude cloud encounters using data obtained between 1975 and 1979 from commercial airliners participating in the Global Atmospheric Sampling Program (GASP) is presented. The statistics are based on three different measures of cloudiness derived from the GASP data set. This climatology depicts the seasonal, latitudinal and altitudinal variation in the cloudiness parameters, as well as differences in the high-altitude cloud structure attributed to cyclone- and convective cloud-generation mechanisms. A qualitative agreement was found between the latitudinal distribution of cloud cover derived from the GASP data and satellite-derived high-altitude cloud statistics available in the literature. Relationships between the three different measures of cloudiness and the relative vorticity at high altitudes, stratified by season, latitude and distance from the tropopause are also presented. In midlatitudes, for example, the average cloudiness, when stratified by the sign of the relative vorticity, exhibits a seasonal cycle with the 1argest differences occurring in the layer 0–1.5 km below the tropopause. Seasonal and latitudinal patterns can also be seen in the other cloudiness parameters.