MagazinePGED

The Jetstream and The Weather in the UK
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In order to understand the jet stream you need
to have some idea of what is occurring in the
atmosphere nearest the ground; known as the
troposphere. It is also helpful to understand the
formation of Hadley, Ferrel and Polar cells and how
The Coriolis Effect influences these tropospheric
cells. The jet stream flows in both hemispheres
around the earth, but for ease this article will
consider the Northern Hemisphere only.
The jet stream is a strong flowing ribbon of air
that flows around our planet high up in the
atmosphere, at around the level of the tropopause.
Situated between the troposphere and the
stratosphere, the Jet Stream is approximately 11
kilometres above the surface of the Earth at the
poles and around 17 kilometres above the surface
of the Earth at the equator. The jet stream flows
at around 160kmph (100mph). We often hear that
the jet stream is responsible for influencing the
weather in the UK, so it is natural to wonder
what causes the jet stream and why it has such
an influence on the weather we experience on
the ground.
Jet streams form and are strongest where variable
air temperature gradients are steepest. This is
normally seen in two zones:
•The boundary between the polar and
mid latitude air . The Polar Front Jet or Polar jet
•The boundary between the mid latitude
air and tropical air . The Subtropical Jet.
Both these jets are separate entities but they
can join up from time to time across an area of
the earth. The polar jet is the strongest as the
temperature gradient across Polar and mid latitude
regions is greatest. This is increased again
in the Northern Hemisphere during winter. Both
the subtropical jet and the polar jet travel from
west to east and both would travel uniformly
and evenly around the earth if it wasn.t for other
influencing factors
See Fig 1. below for an example of a typical jet
stream chart across the earth.
Typical JetStream
Fig 1: The jet stream is shown in red at its strongest
point fading to yellow at its weakest. The STJ
is the area where the subtropical jet is flowing,
the PJ is an area where the polar jet flows and
the P&STJ shows where the jet streams have
combined.
Influencing factors on the Jet Stream flow
The factors that influence the flow of the jet
stream are the landmasses and the Coriolis effect.
Landmasses interrupt the flow of the jet stream
through friction and temperature differences,
whilst the spinning nature of the earth accentuates
these changes. So the jet stream meanders
across the earth, like a river meanders before
it reaches the sea. The meandering sections of
the jet stream continue to change as they interact
with landmasses once again, creating an
ever-changing state of flux and subsequent temperature
differences. In winter the temperature
of the stratosphere can also have an effect on
the strength and position of the jet stream. The
cooler the polar stratosphere, the stronger the
polar/ tropical differential becomes; encouraging
the jet stream to gain in strength. The warmth of
the landmasses and oceans (such as the El Nino
Southern Oscillation) can also have a bearing on
the strength and amplitude of the jet stream.
Jet Stream Variables
The strongest areas of the jet stream are known as
jet streaks. These are areas where the jet stream
has increased in speed by as much as 100kmph.
A typical jet streak is 160km wide, 2-3 km thick
and lasting several hundred km in length. The
strongest jet streams are seen where the upper
air temperature differentials are greatest, the
weakest jet streams appear when the opposite
is true. Strong jet streams tend to have very little
meandering associated with them whereas
weaker jet streams have a considerable amount
of meandering associated with them.
How does the jet stream affect the weather
in the UK?
To try explain how the jet stream affects the
weather in the UK, it is worth going back to
the jet stream chart seen in Fig 1, now zoom in
to the jet stream analysis over the Atlantic as
seen in Fig 2.
Jet Analysis
From Fig 2. you can see that the polar front
jet meanders across the country in a wave
like pattern. These waves introduce pockets of
colder air southwards and warmer northwards.
The temperature change is demonstrated
by looking at the air temperature at a high
enough altitude in the atmosphere, where
ground and sea temperatures do not affect it
.around 1500 metres in altitude where the air
pressure is around 850 hPa.
Fig 3. shows how these temperatures match
the fluctuations in the jet stream.
From looking at Fig 3 and the 850-hPA temperatures
you can see the clear boundaries between
the cold and warm air masses, which is being
divided by the polar jet. You can also see the
demarcation between warm and hot air that the
sub tropical jet is demarcating. This demonstrates
the pattern of the jet stream, showing how it is
linked to and by the difference in warmer and
cooler air masses. Furthermore, looking back at
FIG 2, the wave pattern associated with the polar
jet has distinct peaks (ridges) and troughs. It is
no surprise to find that when we overlay the jet
stream with a chart showing sea level pressure,
that the ridges occur where sea level pressure is
highest and the troughs occur where sea level
pressure is lowest (as FIG 4 demonstrates). This
is because air rises where troughs are situated
and sinks where ridges are occur. Rising and
sinking parcels of air will therefore determine
the type of weather that a region experiences.
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