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Chapter 4 – Air Masses, Fronts and Cyclones
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Overview
 Air masses.
 Types.
 Source regions.
 Modification.
 High and low pressure systems.
 Characteristics.
 How they move.
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Air Masses
 Air masses are high pressure areas with fairly uniform
characteristics.
 Temperature.
 Humidity.
 They are labeled by source region:
Classification
 Continental (c) or maritime (m).
Continental
(dry)
 Tropical (T), Polar (P) or Arctic (A).
Maritime
(moist)
 Types are:
 cP (dry and cold), cT (dry and warm).
 mP (moist and cold), mT (moist and warm).
 cA—continental Arctic (dry and very cold).
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Polar (cold)
Tropical
(warm)
cP
cT
mP
mT
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Sources of Air Masses
 Source regions impart
characteristics to the air mass
within a few days.
 As they move, they are
modified by the surface over
which they pass.
 Temperature increased or
decreased.
 Moisture added or removed.
 Fronts are where they meet.
Courtesy of NOAA
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High and Low Pressure Systems
 High pressure systems (highs) are characterized by:
 Stable, sinking air with surface divergence.
 Light winds that increase with distance from the center.
 Weather is clear to partly cloudy with no precipitation.
 Clear skies at night can produce dew, frost or fog.
 Low pressure systems (lows) are characterized by:
 Unstable, rising air, with surface convergence.
 Strong, gusty winds that increase closer to the center.
 Unsettled, cloudy weather, often with precipitation.
 Can have steady rain, showers, or thunderstorms.
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Movement of Highs and Lows
 Highs and lows are steered by the winds in the upper
atmosphere, at and above about 18,000 feet (500 mb).
 Divergence or Convergence aloft affects highs and lows.
 Divergence aloft (spreading isoheights or winds speeding up)
causes rising air, forming or deepening lows.
 Convergence aloft (tightening isoheights or winds slowing down)
causes falling air, forming or strengthening highs.
 Temperature advection aloft also affects highs and lows.
 Positive temperature advection lowers surface pressure.
 Negative temperature advection raises surface pressure.
 As the wind pattern aloft moves, the highs and lows move with
it.
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Movement of Highs and Lows
©2008, US Power Squadrons. Reprinted with permission
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Overview
 Fronts.
 Types.
 Symbols.
 Characteristics.
 Storm systems (extra-tropical cyclones)
 Structure.
 Characteristics.
 Cloud types.
 Formation.
 Movement.
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Fronts
 Fronts are the boundaries between different air masses.
 There are four different types (one with subtypes).
 Types are based on which air mass is advancing.
 Cold fronts are where cold air is pushing under warm air.
 Warm fronts are where warm air is riding over cool air.
 Stationary fronts have very little movement.
 Occluded fronts are where a fast-moving cold front overtakes a
warm front. It involves three air masses: cold, cool, and warm.



In cold occlusions, the cold air pushes under the cool air.
In warm occlusions, the cool air overrides the cold air.
In both cases, the warm air is raised off the ground.
 The next few slides expand on these definitions.
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Fronts on Surface Charts
 The four types of fronts are shown how they appear on
surface charts. Also included is a trough (or trof). Fronts
move in the direction of the pips.
Cold
Warm
Stationary
Occluded
Trof
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Cold Fronts
Cold fronts are fast moving and aggressively push under warm air,
making it unstable and producing convective activity. They have
strong winds and showery precipitation.
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Warm Fronts
 Warm fronts are gentler and have steady precipitation and
stratiform clouds. Depending on the temperature gradient, it
can be rain, snow, sleet, or freezing rain.
Cool
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Comparison of Cold and Warm Fronts
 This figure compares cold and warm fronts—their clouds,
slopes, and extent.
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Stationary Fronts
 Stationary fronts remain in nearly the same place, sometimes
for days, wandering back an forth. Over time, the weather
comes to resemble that of a warm front.
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Occluded Fronts
 Occluded fronts involve
three air masses.
 Whether the occlusion is
warm or cold depends on
the relative arrangement
of the cold, cool, and
warm air masses.
 Top figure is a warm
occlusion.
 Bottom figure is a cold
occlusion.
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Cyclones
 Cyclone means the system
has cyclonic circulation.
 In the mid-latitudes, they
are more properly called
extra-tropical cyclones and
almost always have fronts .
 This is to distinguish them
from tropical cyclones,
which are related to
hurricanes.
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How is the Cyclone Moving?
 In this picture, note how
the low moves relative to
an observer.
 If the low is passing to the
south of the observer, the
winds will back.
 If the low is passing to the
north of the observer, the
winds will veer.
 Remember Buys-Ballot.
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The Formation of Lows
 Cyclogenisis is the formation of cyclones.
 There are two theories of how cyclones form:
 The classic theory involves waves forming on the polar front.


The polar front resides between the polar easterlies and the
prevailing westerlies.
Disturbances on the polar front develop into cyclones because
the winds are flowing in opposite directions.
 The modern theory is based on the upper air convergence
and divergence discussed earlier. The polar jet stream is over
the polar front and divergence aloft forms lows.
 The next slide shows a series of images depicting the classic
theory.
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Classic Cyclogenisis Theory
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Modern Cyclogenisis Theory
 As explained earlier, divergence aloft causes rising air, forming
low pressure areas (cyclones).
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Storm Tracks
 There are three basic source
regions for cyclones.
 The Pacific Ocean.
 The Midwest.
 The Gulf.
 Storms tend to follow the
tracks shown here.
 mP comes from the Pacific.
 cP comes from the Midwest.
©1997, USA Today. Reprinted with permission
 mT comes from the Gulf.
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Summary (1 of 2)
 Air masses are pools of air with similar characteristics.
 Temperature—tropical, polar or arctic.
 Moisture—continental or maritime.
 Source regions determine type: cP, cT, mP, mT, cA.
 They are modified as they move over new areas.


Addition or subtraction of moisture.
Heating or cooling.
 A front is where two or more air masses meet.
 Cold front—cold air pushes aggressively under warm air.
 Warm front—warm air rises over cool air.
 Stationary front—little movement.
 Occluded—cold front overtaking warm front.
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Summary (2 of 2)
 High pressure areas are air masses with anti-cyclonic rotation.
 Low pressure systems are extra-tropical cyclones.
 They are where fronts meet.
 There are other lows that are due to convergence or heating.
 Pressure systems are steered by upper level winds and
modified by upper level convergence or divergence.
 If a low passes to the south, the wind backs.
 If a low passes to the north, the wind veers.
 Lows form by cyclogenisis.
 Divergence aloft lowers surface pressure.
 Circulation develops, forming a wave on the polar front.
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Chapter 4 Questions
QUESTION
ANSWER
In high pressure systems, the air moves in
what manner?
Spirals clockwise out of the center.
In low pressure systems, the air moves in
what manner?
Spirals counter-clockwise into the
center.
Precipitation ahead of and on a cold front
is typically:
Short in duration and showery.
When a cold front passes, the wind does
what?
Veers sharply.
Cyclones often form on the polar front
because:
The prevailing westerlies and polar
easterlies oppose each other.
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Chapter 4 Questions
QUESTION
ANSWER
Clouds ahead of a warm front are typically
of what type?
Stratiform.
Clouds ahead of a cold front are typically
of what type?
Cumuliform.
When the wind speeds up with distance, it
is said to be:
Diverging.
Divergence aloft encourages cyclone
development because it:
The symbol for a cold front is:
Supports convection and surface
convergence.
A line with triangular pips.
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Chapter 4 Questions
QUESTION
ANSWER
The symbol for a warm front is:
A line with semi-circular pips.
A line with triangular and semi-circular
pips pointing in different directions marks:
A stationary front.
A line with triangular and semi-circular
pips pointing in the same direction marks:
An occluded front.
A cP air mass from Canada, moving
through the central United States will be
modified in what way?
Warmed from the surface upward.
The paths of cyclones through the U.S. is
determined by:
Winds at and above 18,000 feet.
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End of Chapter 4
 Are there any questions?
 Chapter 5 covers Clouds,
Precipitation, and Optical
Effects.
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