Chapter 12
Polar Front Theory
Polar front is a semi-continuous
boundary separating cold, polar air from
more moderate mid-latitude air
 Mid-latitude cyclone (wave cyclone)
forms and moves along polar front in
wavelike manner
 Frontal wave, warm sector, mature
cyclone, triple point, secondary low,
family of cyclones

Where do mid-latitude cyclones tend
to form?
Lee-side lows (cyclogenesis)
 Nor’easters
 Hatteras low
 Alberta Clipper
 Explosive cyclogenesis, bomb

Where do mid-latitude cyclones tend
to form?

Topic: Northeasters
 Mid-latitude cyclones that develop or
intensify off the eastern seaboard of North
America then move NE along coast
Vertical Structure of Deep Dynamic
Lows
Dynamic low = intensify with height
 When upper-level divergence is stronger
than surface convergence (more air is
taken out of the top than the bottom)
surface pressure drops and low
formation
 Topic: Convergence and divergence

 Convergence between ridge and trough,
divergence between trough and ridge
Upper Level Waves and Mid-latitude
Cyclones
Longwaves and shortwaves
 Barotropic vs. baroclinic
 Cold and warm air advection

The Necessary Ingredients for
Development of Mid-latitude Storm
Baroclinic instability
 Upper-Air Support: the overall effect of
differential temperature advection is to
amplify the upper level wave; cut-off low
 Role of the Jet Stream: the polar jet
stream removes air from surface
cyclone and supplies air to surface anticyclone

The Necessary Ingredients for
Development of Mid-latitude Storm

Topic: Jet Streaks and Storms
 Entrance and exit regions associated with
divergence and convergence, right exit
allows divergence.
Conveyor Belt Model: air constantly
glides through storm; warm, cold, and
dry conveyor belts
 March Storm of 1993

Vorticity, Divergence and
Development of Cyclones
Vorticity is a measure of the spin of
small air parcels
 Positive: cyclonic, negative: anticyclonic
 Divergence aloft causes and increase in
the cyclonic vorticity of surface cyclones
= cyclogenesis and upward air
movement

Vorticity on a Spinning Planet
Earth’s vorticity always positive
 Relative vorticity: curvature + shear,
trough: cyclonic, ridge: anticyclonic
 Absolute vorticity = Earth + relative
 An increase in absolute vorticity is
related to upper level convergence
 A decrease in absolute vorticity is
related to upper level divergence
 Vorticity maxima/minima

Vorticity on a Spinning Planet

Topic: Vorticity and Longwaves
 Longwaves develop in upper-levels due to
the conservation of absolute vorticity.

Putting It All Together
 Forecasters review 200mb, 500mb, and
surface maps to examine pressure,
convergence, vorticity, and advection
Polar Lows

Storms that develop over water behind
(poleward of) main polar front.
 Comma cloud, eye
 Warm central core, strong winds, heavy
showery precipitation.
 Arctic front = baroclinic instability