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Jet streams are narrow, concentrated bands of meandering high velocity geostrophic streams, bounded by low speed winds and are a part of upper level westerlies.
Jet streams move in the direction of westerlies in a wavy, irregular manner with a pole-ward or equator-ward component. Jet streams are characterised by circumpolar orientation.
There are two main jet streams — subtropical jet stream at lower latitudes and polar front jets at mid-latitudes. The primary reason for generation of jet streams is the north-south temperature gradient which creates a corresponding pressure gradient. This pressure gradient increases with altitude and creates high velocity winds at higher altitudes.
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According to one view, the sub-tropical jet stream is produced by the earth’s rotation. At the equator, the rotation produces greatest velocity in the atmosphere. As a result, the rising air which spreads out northwards and southwards, moves faster than the latitudes over which it is blowing. It is deflected to the right in the northern hemisphere and to the left in the southern hemisphere, and at about 30° latitude, it becomes concentrated as the subtropical jet streams.
The polar front jet is produced by a temperature difference and is closely related to the polar front. It has a more variable position than the sub-tropical jet. In summer, its position shifts towards the poles and in winter towards the equator.
The jet streams have an average velocity of 120 kilometres per hour in winter and 50 km per hour in summer. These jet streams also have cores where the speed is much greater. They run for thousands of kilometres in length, may range from 40 to 160 kilometres in width and 2-3 km in depth and move at an altitude of 9-12 km at lower latitudes and lower still at higher latitudes.
Their strength rises proportionately with the temperature gradient at lower levels. Usually there are severe storms when jet streams interfere with surface wind systems. Jet streams are also used by aviators if they have to fly in the direction of the flow of the jet streams.
Influence of Jet Streams on World Weather:
1. Jet streams help in maintenance of latitudinal heat balance by mass exchange of air.
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2. Polar front jets influence the mid-latitude weather disturbances.
3. Jet streams also influence the path of cyclones.
4. They have an influence on distribution of precipitation by the temperate cyclones.
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5. Jet streams also exercise an influence on movement of air masses which may cause prolonged drought or flood conditions.