I—Strong Winds in the Upper Atmosphere Above 15,000 ft.

Extract

This paper gives a description in very general terms of where and at what seasons may be found the strong winds which occur in the upper atmosphere above 15,000 ft. and discusses briefly the clear air turbulence which is sometimes found in and near these strong winds.

Jet streams of temperate latitudes. Navigators are now well aware of the comparatively narrow bands of strong winds, known as jet streams, which occur in the upper troposphere in temperate latitudes. These jet streams, which can stretch for over 2000 miles on occasions, are very variable in position and in strength. It is probably true to say that there is a jet stream with wind speeds exceeding 60 kt. somewhere between Jan Mayen (N. 71°) and Gibraltar (N. 36°) every day, except in quiet summer weather and exceptionally on quiet days in other seasons. Fig. 1, from Durst and Davis shows a typical jet stream; these winds are associated with fronts though the fronts are often of the dry type with very little associated weather. For example, the strong northerly jet stream over the British Isles on 14 November 1949, with speeds of over 150 kt., occurred over a surface anticyclone; it was in this jet stream that the Comet I, on an early flight, experienced very severe turbulence. The level of the maximum wind in these jet streams is about 30,000 ft. though the exact height varies from case to case. Since they are so variable in position and strength the navigator must take particular note of them and a close liaison with the forecaster is clearly essential.