weather

What is the jet stream and how does it affect the weather?

jet streams are another name for fast

flowing currents of air there are many

different jet streams that exist at both

high and low levels in the atmosphere

each has an important role to play in

the weather experienced on earth the

major jet streams that circle the earth

are found just below the tropopause at a

heights ranging from 9 to 16 kilometers

and can reach speeds over 200 miles per

hour they're hundreds of kilometres wide

by only a few kilometers deep so often

described as a ribbon of very strong

winds the major jet streams are the

polar front and subtropical jets these

occur in both the northern and southern

hemispheres and a part of the larger

global circulation the polar front jet

occurs over mid latitudes and strongly

influences the weather over the UK in

Europe the polar front jet is a type of

thermal wind that arises due to the

strong temperature contrasts between

cold polar air and warm tropical air to

explain this further imagine two columns

of air one in the cold air to the north

of the jet and one in the warm air to

the south the top of each column is

bounded by the tropopause a shorter

column of cold air exerts the same

surface pressure as a taller column of

wor air this is because in the cold air

column the air is more dense

this causes atmospheric pressure to

decrease more quickly with Heights in

the warm air column pressure does not

decrease as rapidly with height because

the warmer air is less dense

the tropopause in both columns is at the

same atmospheric pressure so if you were

to climb to the same heights in both

columns you would find that you would be

at a lower atmospheric pressure in the

cold air column and a higher atmospheric

pressure in the warm air column the

pressure difference caused by this

temperature gradient produces a pressure

gradient force the pressure gradient

force acts from high to low pressure so

theoretically in the Northern Hemisphere

air would flow from south to north

but the Coriolis force resulting from

the Earth's rotation causes the air to

move to the right of the direction of

motion in the Northern Hemisphere and to

the left of the direction of motion in

the southern hemisphere in both

hemispheres the jet stream flows

parallel to this temperature gradients

moving from west to east theoretically

this jet encircles the earth in a

continuous line but in reality it is

more broken up as differential heating

of land sea masses lead to west to east

temperature contrasts as well as north

to south as the polar front Jets forms

due to the temperature contrast the

stronger the temperature gradient

the stronger the Jets this means that

the jet is stronger in the winter than

the summer as the poles call during the

winter months increasing the temperature

contrast in the northern hemisphere the

jet tends to be further south in winter

and further north in summer this is due

to the tilt of the earth and that in the

winter the northern hemisphere is tilted

away from the Sun

and in the summer it is tilted towards

the Sun mid latitude low pressure

systems occur on the polar or northern

side of the Jetstream while more settled

warmer conditions are found to the south

this gives us the wet and windy weather

we often see in the UK during the winter

months as low pressure systems are

steered towards us it can also lead to

some disappointingly wet summers with

the Jetstream remains to the south of

the UK a stationary jet stream pattern

will bring frequent low pressure systems

to the same region when warm air moves

further north and normal or cold polar

air moves further south this can change

the prevailing west to east jet stream

pattern

this causes the jet stream to buckle

driving depressions towards different

regions or blocking their movements

altogether

a straight West East flowing jet stream

won't have much impact on the

development of new weather systems or

strengthened pre-existing ones when a

jet stream meanders north and south air

accelerates and decelerates around the

bends and the flow these areas of

changing speeds are our development

areas where the air is accelerating more

air is leaving than entering a certain

point this means that the air is being

depleted at this point to fill the

depletion air from below Rises when air

rises it forms clouds and precipitation

winds at the surface converge to this

rising air column which causes surface

pressure to fall and a low pressure

system may develop or deepen further so

whilst a jet stream is just an area of

fast flowing air it has a fundamental

effect on our weather

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