Jet streams are important in controlling temperature, cloudiness and precipitation at midlatitudes because they induce vertical motions in the troposphere; subsidence usually means fine weather, while uplift can lead to rainfall. The place of greatest uplift is on the poleward side of the exit of a jet streak, called the delta region (Figure 12.15) where two causes of upper-level divergence combine. First, there is that due to acceleration from cyclonic to anticyclonic rotation at the exit of the jet streak, and, second, that caused by the momentum of the jet's air at the exit which generates divergence on the right-hand side because of a temporary excess of leftwards Coriolis acceleration (Note 12.L). The 'delta region' is seen in satellite pictures of clouds to be about 1,000 km across and triangular, like the Greek letter delta.
Uplift due to a jet causes low-level convergence and hence a surface low which may grow into a frontal disturbance and then a storm (Chapter 13). The frontal disturbances are responsible for alternations of cold and warm, wet and dry weather in midlatitudes. The storms have led to the belt of highest jet-stream activity being called the storm track. It is centred at about 47°S in both winter and summer, which is clear of Australia, South Africa and (just) New Zealand, but not South America, which extends to 56°S. Slight movement of the storm track, due to changes in ocean circulation for instance, can significantly alter climates locally.
Jet streaks are also linked with Clear Air Turbulence, a little-understood hazard to aviation (Note 12.M).
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