- 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:
- 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 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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