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Jet Streams

  • Jet streams are high speed winds that occur in narrow bands of upper air westerlies.
  • The width of this air band can be 160-480km wide and 900-2150m thick, with core speed exceeding 300km/hr. such is their strength that aircraft routes which run counter to jet movements are generally avoided.
  • Jets are coincident with major breaks in the tropopause.

Jet streams can be classified as follows:

Polar front jet stream:

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  • this is a thermally induced jet stream and it flows parallel to surface fronts.
  • They flow west to east in a sinusoidal fashion.
  • It is strongest at 200-300mb level and swings between 40-60 degree latitude.
  • It is found in both the hemispheres.
  • Its band is non- continuous but flows all-round the year.
  • It can reach up to 160-200 km/hr.

Tropical westerly jet streams:

  • they also flow all-round the year.
  • They flow to conserve the angular momentum in upper atmosphere.
  • They are found at the pole ward limit of Hadley cell around 30 degree N and S latitude.
  • It follows a more fixed pattern than polar jet stream.
  • It is strongest on Indian sub-continent.
  • The maximum speed can reach up to 300km/hr. the subtropical westerly jet do not seem to affect surface weather as much as the polar fronts jets do.

Tropical easterly jet:

  • they are seasonal jet streams flowing east to west.
  • These are only found in northern hemisphere and generates only in summer season.
  • These are also thermally induced.

Geostropic Winds:

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  • Geostropic winds are those which flows parallel to isobars.
  • These are found in upper atmosphere.
  • In upper atmosphere, only two forces are present which influences the direction of wind.
  • They are pressure gradient and Coriolis force.
  • At this height friction is not present.
  • As the pressure gradient acts on a parcel of air, it starts moving from high pressure to low pressure.
  • As it moves, the coriolis acts on it perpendicular to its direction changing its direction.
  • As the velocity of wind increases the coriolis also increases making it more and more parallel to isobars.
  • A time comes when the pressure gradient force becomes equal to Coriolis force, nullifying each other.
  • At this time the wind flows at uniform velocity as net force is zero and it will flow parallel to isobars.

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