Transverse flow at entrance and exit to jet streams

From a statistical examination of wind observations near jet streams, two conclusions are reached: (a) In the jet‐stream entrance region there is evidence of a component of wind towards the left (looking downstream), and this reaches a maximum at the level of the jet centre and falls off above and below. It is probable that this arises from an ageostrophic flow across the contours as a consequence of the acceleration of the air downstream. The transverse flow would be consistent with the existence of a direct thermal circulation either above or below the jet stream, or both. (b) In the exit region the cross‐stream wind component is towards the right (looking downstream), and it reaches a maximum at the level of the jet centre and falls off above and below. This probably arises from an ageostrophic motion across the contours as a result of the deceleration of the air downstream. It would be consistent with an indirect thermal circulation either above or below the jet stream, or both. For a sample of observations over the British Isles, the jet axis in entrance regions is in the mean about 2,000 ft (27 mb) higher than in exit regions.