Understanding Weather
and Climate
3rd Edition
Edward Aguado and James E. Burt
Anthony J. Vega
Part 4. Disturbances
Chapter 10
Mid-latitude Cyclones
Introduction
Bjerknes, the founder of the Bergen school of
meteorology, developed polar front theory to describe
interactions between unlike air masses and related aspects
of the mid-latitude cyclone
The Life Cycle of a Mid-latitude Cyclone
Cyclogenesis typically begins along the polar front but may initiate
elsewhere, such as in the lee of mountains
• Minor perturbations occur along the boundary separating colder polar
easterlies from warmer westerlies
• A low pressure area forms and due to the counterclockwise flow
(N.H.) colder air migrates equatorward behind a developing cold front
• Warmer air moves poleward along a developing warm front (east of
the system)
• Clouds and precipitation occur in association with converging winds
of the low pressure center and along the developing fronts
Cyclogenesis
Mature Cyclones
• Well-developed fronts circulating about a deep low pressure center
characterize a mature mid-latitude cyclone
• Chance of precipitation increases toward the storm center
– Heavy precipitation stems from cumulus development in association
with the cold front
– Lighter precipitation is associated with stratus clouds of the warm front
• Highly unstable conditions are associated with the warm sector ahead
of the cold front
– Area may produce heavy precipitation and severe thunderstorms
associated with prefrontal waves (squall lines)
• The system is capable of creating snow, sleet, freezing rain, and/or
hail
• Isobars close the low and are typically kinked in relation to the fronts
due to steep temperature gradients
• Winds, spiraling counterclockwise toward the low, change
accordingly as the system, and its associated fronts, moves over
particular regions
A mature mid-latitude cyclone
A mature mid-latitude cyclone,
lifting processes, and cloud cover
Two examples of
mid-latitude cyclones
Occlusion
• When the cold front joins the warm front, closing off the warm sector,
surface temperature differences are minimized
• The system is in occlusion, the end of the system’s life cycle
Evolution and Movement of Cyclones
• A hypothetical mid-latitude cyclone may develop as a weak
disturbance east of Japan and travel eastward, guided by upper air
trajectories
• The system may bring rain to western North America and snow to
high elevations
• On the lee side of the Rocky Mountains, the system may undergo
strengthening, causing blizzard conditions in the central and
northeastern U.S.
• Occlusion typically occurs in the western North Atlantic
• For particular locations, weather conditions may progress from clear
to cloudy with cloud cover thickening and lowering
• Eventually, light precipitation may begin with warm front
advancement
• Winds, originally easterly, shift to southeasterly, then southwesterly
with warm front advancement
• Heavy clouds and precipitation advance with cold front approach
• Temperature and humidity plummet with cold front passage
Processes of the Middle and Upper Troposphere
Carl Rossby mathematically expressed relationships between midlatitude cyclones and the upper air during WWII
Rossby Waves and Vorticity
• The rotation of air, or vorticity, may be viewed as either being
absolute, the overall rotation, or relative to the Earth’s surface
• Air which rotates in the direction of Earth’s rotation is said to exhibit
positive vorticity
• Air which spins oppositely exhibits negative vorticity
• In relation to the upper air, maximum and minimum vorticity occurs
in relation to troughs and ridges, respectively
• Vorticity changes in the upper atmosphere lead to surface pressure
changes
– Decreasing vorticity in the zone between a trough and ridge leads to
upper air convergence and sinking motions through the atmosphere,
which supports surface high pressure areas
– Increasing vorticity in the zone between a ridge and trough leads to
upper air divergence and rising motions through the atmosphere, which
supports surface low pressure areas
Relative vorticity
Vorticity around a Rossby
wave
Changing vorticity along a Rossby wave
Convergence and divergence
along a Rossby wave
Values of absolute vorticity
on a hypothetical 500 mb map
Changes in vorticity through
a Rossby wave
The Effect of Fronts on Upper-Level Patterns
• Interactions between the upper air and surface and vice versa helps
establish and develop mid-latitude cyclones
• Thermal differences across cold and warm fronts lead to upper
atmospheric pressure differences due to density differences which
equate to air temperature as expressed through the hydrostatic
equation
Cold Fronts and the Formation of Upper-Level Troughs
• Upper air troughs develop behind surface cold fronts with the vertical
pressure differences proportional to horizontal temperature and
pressure differences
• This is due to density considerations associated with the cold air
• Such interactions also relate to warm fronts and the upper atmosphere
Temperature variations in the lower atmosphere
lead to variation in upper-level pressure
• Interaction of Surface and Upper-Level Patterns
– The upper atmosphere and the surface are inherently connected and
linked
– Divergence and convergence relate to surface pressure differences in
cyclones and anticyclones, respectively
– Surface temperatures influence vertical pressures and upper atmospheric
winds
– Upper level flow patterns explain why mid-latitude cyclones exist in
addition to aspects of their life cycles
– An example is the typical position of mid-latitude cyclones downwind of
trough axes in the area of decreasing vorticity and upper-level
divergence
Relationships between
a mid-latitude cyclone
and a trough and ridge
• An Example of a Mid-Latitude Cyclone
– April 15 - A mid-latitude cyclone is centered over the upper midwestern
U.S.
– Heavy rains, high winds, and overcast skies dominate the regions near the
central low pressure center
– Recording of wind trajectories at stations throughout the central U.S.
depict the counterclockwise rotation of the system
– The 500 mb chart shows the storm upstream of the trough axis in the
region of decreasing vorticity and upper-level divergence
– April 16 - The northeasterly movement of the storm system is seen
through a comparison of weather maps over a 24-hour period
– Occlusion occurs as the low moves over the northern Great Lakes
– In the upper air, the trough has increased in amplitude and strength and
become oriented northwest to southeast
– Isobars have closed about the low, initiating a cutoff low
– April 17 - Continual movement towards the northeast is apparent,
although system movement has lessened
– The occlusion is now sweeping northeastward of the low, bringing
snowfall to regions to the east
– In the upper air, continued deepening is occurring in association with the
more robust cutoff low
– April 18 -The system has moved over the northwestern Atlantic Ocean,
but evidence persists on the continent in the form of widespread
precipitation
– The upper atmosphere also shows evidence of the system, with an
elongated trough pattern
• Flow Patterns and Large-Scale Weather
– Zonal height patterns obstruct development of surface pressure systems
as vorticity remains constant
– Zonal conditions are indicative of rather benign atmospheric conditions
at the surface, although small scale disturbances may occur
– Meridional conditions actively support surface cyclone development as
vorticity changes appreciably between troughs and ridges
– Large-scale flow conditions in the upper atmosphere may persevere for
long periods, locking in particular weather situations to affected regions
Zonal flow pattern
Meridional flow pattern
The Steering of Mid-Latitude Cyclones
• The movement of surface systems can be predicted by the 500 mb
pattern
• The surface systems move in about the same direction as the 500 mb
flow, at about 1/2 the speed
• Must predict changes in the 500 mb flow patterns in order to correctly
predict surface system movement
• Upper-level winds are about twice as strong in winter than summer
• During winter, net radiation decreases rapidly with increasing latitude,
which creates a stronger latitudinal thermal gradient
• This results in stronger pressure gradients (and winds), resulting in
stronger and more rapidly moving surface cyclones
• Winter mid-latitude cyclones may be grouped by common paths
across North America
– Alberta Clippers are associated with zonal flow and usually produce
light precipitation
– Colorado Lows are usually stronger storms which produce more
precipitation
– East Coast storms typically have strong uplift and high water vapor
content
Typical winter mid-latitude cyclone paths
Migration of Surface Cyclones Relative to Rossby Waves
• Upper-air divergence must be present for a mid-latitude cyclone to
form
• If divergence aloft exceeds surface convergence, the surface low will
deepen and a cyclone forms
• If convergence at the surface exceeds divergence aloft, the low “fills”
• Surface cyclones are pushed along the upper air wind trajectory
• They generally move in the same direction as the 700 mb winds and
at about 1/2 the speed
• The surface low generally moves southwest to northeast relative to
the Rossby wave configuration
• As such, it moves away from the region of maximum divergence
aloft, eventually dissipating as it approaches the upper-level ridge
The Modern View - Mid-latitude Cyclones and Conveyor Belts
• The conveyor belt cyclone model helps describe conditions associated
with mid-latitude cyclones through the entire profile of the
atmosphere
• The warm conveyor belt originates in the lower atmosphere of the
warm sector
– Air flowing toward the storm center is displaced aloft until it overrides
the warm front where it turns to the right (N.H.), becoming part of the
westerly flow aloft
• The cold conveyor belt lies north of the warm front
– It streams westward near the surface toward the surface low, where it
ascends and turns clockwise (N.H.) to become part of the westerly upper
air flow
• The dry conveyor belt originates in the upper troposphere as part of
the normal westerly flow
– Air sinks into the trough only to rise over the region of the surface low
before continuing along its eastward path
– Integral to maintaining separate cloud bands which give the system its
characteristic comma shape
End of Chapter 10
Understanding Weather and
Climate
3rd Edition
Edward Aguado and James E. Burt