Lecture 24. Middle latitude
cyclones.
Polar front theory. Formation and
development of mid-latitude cyclones. Midlatitude cyclones and polar jet stream.
Reading: Ch. 8, 219–227.
Cyclone
• A middle-latitude
low-pressure
system, complete
with fronts and
precipitation
• Polar front theory
of mid-latitude cyclone
life cycle (birth, growth,
and decay) provides
a conceptual way to
describe weather
Polar front theory
Polar front
shear
Formation of an occluded front
Instability forms
Open
frontal wave
wind wave
Most intense
storm
Dissipation
stage
! Cycle duration: a few days to a week
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A “family” of cyclones
in different stages of
development
Typical paths of winter midlatitude cyclones
Vertical structure of cyclones
and cyclone development
• When upper-level divergence
of air around low-pressure
center is stronger than
surface convergence, the
cyclone will intensify, and
vice versa
• When upper-level convergence
of air around high-pressure
center is weaker than
surface divergence, the
anticyclone will intensify, and
vice versa
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Convergence and divergence
• Convergence — the piling up of the air above
some region
• Divergence — the
spreading out of the air
above some region
Cyclogenesis
• Cyclogenesis is the development and
strengthening of a middle latitude storm
system
• The cyclone is said to be deepening
when the surface low pressure in the storm
system keeps decreasing
• The anticyclone is said to be building
when the surface high pressure keeps
increasing
Cyclogenesis (cont’d)
• Building anticyclones are associated with
upper-air convergence being weaker than
surface-air divergence
• Deepening cyclones are associated with
upper-air divergence overwhelming the
surface-air convergence
• Upper-layer convergence and divergence
primarily occur along Polar Front jet stream
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Factors which interact to form cyclone
Surface
Upper level (jet stream)
• Convergence and divergence
• High and low
pressure systems
• Cold and warm advection
• Cold and warm fronts
• Latent heat and associated
energy release
• Precipitation
• Straightened-out jet stream
• Occluded front
Summary of cyclogenesis
Summary of Polar Front theory
• In the middle latitudes (where we are!),
cyclones form along Polar Front jet stream,
the boundary between cold arctic air and
warmer subtropical air
• Major feedback processes:
 Convergence and divergence (surface and upper air)
 Upper-level temperature advection and fronts
! Feedback processes work in both directions
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