The Tricellular Circulation is highly dynamic, varying with the seasons and bringing the
Polar front back and forth across the USA.
RANGE OF THE
POLAR FRONT
THE POLAR FRONT IS THE REGION OF FORMATION OF ANTICYCLONES AND
CYCLONES IN THE MID-LATITUDES. The anticyclone and cyclone are CLOSED
Circulation wind patterns.
The anticyclone pressure gradient forces wind OUTWARD across isobars, and Coriolis
Force forces the wind to turn rightward, while friction causes wind to move from HI to LOW
pressure, resulting in a clockwise rotation and DIVERGENCE.
The cyclonic flow is INWARD in response to the pressure gradient, Coriolis, and frictional forces,
And turns counterclockwise (CCW). Low pressure centers, hurricanes, and tornados are cyclonic.
WIND TYPES AND ORIGINS
1. PRESSURE GRADIENT WIND
ORIGIN: PRESSURE DIFFERENCES
DIRECTION: ACROSS ISOBARS
VELOCITY: TYPICALLY 10-20 MPH (5-10 MPS)
2. GEOSTROPHIC WIND
ORIGIN: PRESSUREGRADIENT AND CORIOLIS FORCE
DIRECTION: PARALLEL TO ISOBARS
VELOCITY: SIMILAR TO PRESSURE GRADIENT WIND
3. GRADIENT WIND
ORIGIN: PRESSURE GRADIENT AND CORIOLIS
FORCE,
BUT WITH AIR MOVING AROUND CLOSED
ISOBARS
DIRECTION: AROUND CLOSED ISOBARS
VELOCITY: SIMILAR TO GEOSTROPHIC WIND, BUT
HIGHER AROUND HIGH PRESSURE CENTER
THAN AROUND LOW PRESSURE CENTER
(EXCEPT FOR EFFECTSOF GREATER PRESSURE
GRADIENTS IN THE CASE OF A TORNADO OR
HURRICANE.
Typical pattern of a mature mid-latitude Northern hemisphere cyclone, showing isobars, and
wind velocities. The pressure gradient shown here is very high, resulting in hurricane force wind.
DEFINITION:
BAROTROPIC REGION:
A REGION OF CONSTANT PRESSURE, DENSITY, AND
TEMPERATURE... I.E. WHERE LITTLE IS HAPPENING
DEFINITION:
BAROCLINIC REGION:
A REGION OF VARYING DENSITY, CONSTANT
PRESSURE, BUT CHANGING TEMPERATURE.
BAROCLINIC REGIONS POSSESS POTENTIAL ENERGY
THAT, WHEN RELEASED, CAN DRIVE THE FORMATION
OF MID-LATITUDE CYCLONES.
DEFINITION:
FRONT
THE BOUNDARY BETWEEN AIR MASSES OF DIFFERING
BAROCLINICITY
I.E. (DIFFERING TEMPERATURE AND DENSITY)
BJERKNES' POLAR FRONT THEORY
IN THE 1920'S, DEVELOPED THE THEORY OF
CYCLONE FORMATION, DEVELOPMENT AND
DISSIPATION.
DEFINITION:
CYCLOGENESIS
THE DEVELOPMENT AND STRENGTHENING OF
A MID-LATITUDE CYCLONE
Relative advection brings air of different temperatures atop one another and results in a change
In the environmental lapse rate, as shown the advection STEEPENS the lapse rate.
The frontal zone tilts TOWARD the cold front as the denser cold air under runs the lighter warm air.
When air velocity differs between regions (V1 and V2), convergence or divergence can occur.
Convergence, as shown here, typically increases lapse rates, making air tend toward instability.
In addition to divergence and convergence, winds can be compressional (katabatic).
Cyclones and Anticyclones work together to create CIRCULATION CELLS in the
Atmosphere. Both are an expression of VORTICITY in the atmosphere, and the
resulting turbulence in the mid-latitudes.
VERTICALLY AS WELL AS HORIZONTALLY, MID-LATITUDE CYCLONES
AND ANTICYCLONES OPERATE LIKE AN ATMOSPHERIC HEAT ENGINE !
CYCLOGENESIS.
As shown here, a
rotation has developed
about a low pressure
center as relative
advection occurs, along
with the effects of
vorticity, producing a
Wave Cyclone.
The Wave Cyclone
seen horizontally along
the black line reveals
The three dimensional
nature of the process.
Specific Clouds are
Associated with
Development of a
Cyclone and its
Passage.
Cloud types observed in association with polar front weather.
Approach of a warm front
Approach of a Cold Front
Cyclogenesis,
Development to
Dissipation
Types of Occluded Fronts
The progression of wave cyclones through maturation and dissipation produces specific
cloud types, wind directions, and precipitation that can aid in the prediction of weather.