The role of meteorology in air operations is becoming increasingly important and an understanding of meteorology is desirable in the consideration of a number of the problems to be faced. The forecasting of winds en route and the use of these forecasts is one such problem, which has been brought into greater prominence by the advent of fast, high-flying aircraft. A great deal of work is being done on upper air winds and it is already clear that curious and significant phenomena sometimes occur. Thus very high winds are sometimes found in belts of limited lateral extent—the so-called “jet stream.” Not only high winds, but high rates of change of wind occur in such regions and they are clearly a potential source of difficulty to the navigator. Apart from the jet stream —a meteorological highlight of recent development—airline operators are also concerned with the problem of route planning.