Temperature frequencies near the flight level of S.S.T. aircraft are here treated with regard to the areas Western Europe–North America–North Pacific. The used values are taken from charts of the 50-mb level (ca. 20 km.). They are divided into three groups: (1) Temperatures higher than standard temperature, i.e. higher than - 56° C (Figs. 1, 2) showing very clearly the big North Pacific warm area, (2) values higher than -46° C (standard temperature plus 10° C) (Figs. 3, 4), mostly expressing the polar warming in summer, and (3) temperatures lower than -65° C (Fig. 5), only occurring in winter and concentrated in the European sector of the Arctic. These effects are described along some flight routes.