Transcript LEGISLATION - Hodder Education

Understanding jet
streams
Understanding jet streams
What are jet streams?
• Jet streams are narrow, fastmoving air currents (winds).
• They are found at high altitude,
at the boundary between the
troposphere and
stratosphere.
• It might be useful to think of jet
streams as almost like ‘rivers’
of very fast winds in the
atmosphere.
• Jet stream winds can blow at
speeds from 50 mph to over
250 mph.
Understanding jet streams
Jet stream locations
• There are four jet streams
• The two polar jet streams are
found at latitude 50–60N and 50–
60S.
• The two sub-tropical jet streams
are found about 30 north and
south of the equator.
• The northern polar jet stream,
usually found at an altitude of 8–
11 km, has a major influence of
the weather experienced by the
UK.
• Sub-tropical jet streams are
weaker than polar jet streams.
Understanding jet streams
The northern polar jet stream
• This northern polar jet stream always
blows from west to east.
• It follows a circular path around the
globe.
• However, the jet stream meanders
north and south in a series of huge
waves known as Rossby waves.
• North of the jet stream position is cold
polar air, but south of the position is
warm sub-tropical air.
• Shifts in the position and size of Rossby
waves means shifts in the position of
cold or warm air over a particular
location.
Two different
Rossby wave
patterns.
In the lower
example, the
waves are
much more
pronounced
allowing warm
air to move
north, and
cold air to
sink south.
Understanding jet streams
• Winds are caused by differences in air
pressure between two locations.
Causes
• Air flows from areas of high air pressure to
areas of low air pressure.
• Air-temperature differences influence air
pressure, because cold air is more dense
than warm air.
• Air temperature and air pressure
differences are very large at the polar
front.
• This is the boundary zone between cold,
dense air over the poles andwarm, and less
dense air from the tropics.
• It is at this location that the strongest jet
stream is found.
Image adapted from: http://apollo.lsc.vsc.edu/classes/met130/notes/chapter10/jet_streams.html
Understanding jet streams
Variability
• The polar jet stream’s position is very
variable, and cannot be predicted.
• Generally, it is over the UK in the
winter, and north of the UK in the
summer.
• However, this can and does change.
• El Niño and La Niña events can also
influence the position of the polar jet
stream, contributing to seasonally
unusual weather patterns.
• The jet stream can even split,
temporarily, into two arms.
Different positions of the polar jet
stream in relation to the UK
Understanding jet streams
Influence on the weather
• The polar jet stream is strongly linked to depressions – areas of low
pressure — whose fronts bring rain to the UK.
• If the jet stream sits over the UK for an extended period it can mean a long
period of wet weather — as front after front is carried in from the Atlantic
(winter 2013–14)
• If the jet stream moves north of the UK for a long period it can lead to
drought and/or heatwave conditions (summer 1976 and 2003).
• ‘Washout’ summers are caused by the jet stream being further south than
expected.
Understanding jet streams
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