The uniform flow of the upper-air westerlies is frequently disturbed by the formation of large undulations, called upper-air waves (or Rossby waves). The waves develop in a zone of contact between cold, polar air and warm, tropical air.
It is by means of the upper airwaves that warm air of low latitudes is carried far north at the same time that cold air of Polar Regions is brought equatorward. In this way horizontal mixing, or advection, develops on a gigantic scale and serves to provide heat exchange between the equatorial region of energy surplus and the polar regions of energy deficit.
Associated with the development of such upper airwaves at altitudes of 30,000 to 40,000 ft (10,000 to 12,000 m) are narrow zones in which wind streams attain velocities up to 200 to 250 knots (350-450 km per hr). This phenomenon. named the jet stream, consists of pulselike movements of air following a broadly curving track. In cross section the jet may be likened to a stream of water moving through a hose, the centre line of highest velocity being surrounded by concentric zones of less rapidly moving fluid.
The jet stream is an important factor in the operation of jet aircraft in the range of their normal cruising altitudes. In addition to strongly increasing or decreasing the ground speed of the aircraft, there is a form of air turbulence that at times reaches hazardous levels. This is clear air turbulence (CAT); it is avoided when known to be severe in intensity.
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