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Chapter 8 –
Part 1
The Dynamic
Development
of High and
Low Pressure
Systems
Extratropical cyclones are the parent storms of many types of hazardous weather
•These cyclones (low
pressure systems) result
from accelerations created
by imbalances between the
pressure gradient force and
the Coriolis force, primarily
at the level of the jet stream.
•Today we will look at how
the jet stream can spawn
high and low pressure
systems at the surface
Extratropical Cyclone near Iceland
Geostrophic Balance and the Jet Stream
Jetstream – a narrow band of strong winds that encircle the earth in the midlatitudes. Produced by a strong PGF and balanced by Coriolis force.
The jetstream is typically 200 to 300 miles wide and can be located between
250 mb and 500 mb (typically ~300mb)
Polar
Up to 3 jetstreams can exist
between the equator and the pole
(they can interact as well):
•Subtropical jetstream
•Polar jetstream
•Arctic jetstream
We will generally consider one
main jet for this class
The large Equator to
Pole temperature
gradient is what
produces large
pressure gradient that
leads to the jet stream.
The exact location of
the jet stream is
influenced by large
temperature gradients
found near the surface
of the midlatitudes
WARM
COLD
COLD
COLD
WARM
WARM
The variations in surface temperature gradients worldwide help make the jets
wavy!
Convergence – a net inflow of air
molecules into a region of the
atmosphere
Convergence within an air column is
always associated with increasing
surface pressure, since the mass per
unit area, or weight of the column,
will increase with time.
Divergence – a net
outflow of air molecules
from a region of the
atmosphere (the opposite
of convergence)
Divergence within an air
column is always
associated with
decreasing surface
pressure, since the mass
per unit area, or weight of
the column, will decrease
with time.
Faster
Slower
Faster
OK, so now we’ve seen what convergence and divergence does
to surface pressure
Let’s now look first at how the upper atmosphere can cause
convergence and divergence at upper levels, which leads to
changes in surface pressure
https://youtu.be/hW57tLMcqt4
What force is
responsible for throwing
the guy off the merrygo-round?
The centrifugal force!
Gradient Wind
Balance
A 3 way balance
between Coriolis,
PG, and
centrifugal forces
For flow around a high (clockwise in NH), the
Coriolis force has to balance both PG & centrifugal
forces. The only way to increase Coriolis at one
location is to increase the windspeed!
HIGH
flow
Gradient Wind
Balance
A 3 way balance
between Coriolis,
PG, and
centrifugal forces
For flow around a low (counterclockwise in NH), the
Coriolis force gets the help of the centrifugal force
to balance the PGF. This means Coriolis force can
be relatively small, and thus the windspeed slows!
flow
LOW
Its important to note that any cyclonic flow (counterclockwise for
NH) will be slower than the geostrophic value and any anticyclonic
flow (clockwise for NH) will be faster than the geostrophic value.
If the jet turns counterclockwise, it will slow down (NH)
If the jet turns clockwise, it will speed up (NH)
HIGH
LOW
Flow around a trough is cyclonic flow and flow around a ridge is anticyclonic flow.
HI
LOW
LOW
•Air flows slow around trough and fast around ridge
•That means air must speed up moving from trough and ridge (just like the start of a
marathon)
•That means air must slow down moving from ridge to trough (just like cars at a stop light)
•Thus you get air converging moving from ridge to trough, and air diverging moving from
trough to ridge
Flow around a trough is cyclonic flow and flow around a ridge is anticyclonic flow.
•Jetstreak – regions of exceptionally strong winds in the jetstream
•Jetstreaks occur where the isobars (or height contours) are very closely spaced.
•At A, PGF=CF, and air travels in
straight line at constant speed
•From A to B, PGF > CF, so air
accelerates and heads north a bit.
• Since speed increases, CF
increases, which will again
balance the PGF at C.
•From C to D, PGF decreases but
wind is already moving fast,
which allows CF to be larger than
PGF. Flow will turn to right and
slow down as a result.
•At E, CF decreases since wind
decreased. PGF=CF and flow is
geostrophic and straight again.
PGF=CF
PGF > CF
CONVERGENCE
DIVERGENCE
PGF < CF
DIVERGENCE
CONVERGENCE
PGF=CF
The net result of these changes in the balances of PGF and CF is the pattern of convergence and
divergence around a jetstreak shown below
Combined Effect of Curvature and Jetstreaks
What happens when a jetstreak and curved flow occur in the same location?
Clicker Session
3 points per question for correct answer
1 point per question for participation
Where would you expect the surface pressure to decrease most rapidly?
Where would you expect the surface pressure to increase most rapidly?
Discuss with your Learning Assistants and/or neighbors!
Clicker 1
Where would you expect the surface pressure
to decrease most rapidly?
A. Upper left (two C’s)
B. Upper right (two D’s)
C. Lower right (C and D)
D. Lower left (D and C)
Clicker 2
Where would you expect the surface pressure
to increase most rapidly?
A. Upper left (two C’s)
B. Upper right (two D’s)
C. Lower right (C and D)
D. Lower left (D and C)
Where would surface pressure rise most rapidly in this
scenario?
Where would surface pressure fall most rapidly in this
scenario?
Discuss with your Learning Assistants and/or neighbors!
Jet streak
NH
1
3
9700m
2
4
9750m
Ridge
9800m
9850m
Trough
9900m
Trough
Where would surface pressure rise most rapidly in this scenario?
A.
B.
C.
D.
Section 1
Section 2
Section 3
Section 4
Jet streak
NH
1
3
9700m
2
4
9750m
Ridge
9800m
9850m
Trough
9900m
Trough
Where would surface pressure fall most rapidly in this scenario?
A.
B.
C.
D.
Section 1
Section 2
Section 3
Section 4
Jet streak
NH
1
3
9700m
2
4
9750m
Ridge
9800m
9850m
Trough
9900m
Trough
Jet streak
NH
3
1
D C
2
C D
9700m
4
9750m
D D Ridge C C
9800m
9850m
Trough
9900m
Trough
Explanation:
•From curvature you have divergence left of ridge and convergence right of ridge.
•From the jet streak, you have divergence at the right entrance and left exit, and
convergence at the left entrance and right exit.
Often, the jet stream splits and you can get 2 jet streaks adjacent to one another
Just considering the jet streaks (not curvature), where would you expect to
find the surface pressure dropping most rapidly?
A
B
C
D
E
Explanation
D
C
C
D D
C
C
D
Have a great weekend!