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Chapter 8: Air Masses and Fronts
Introduction
• Air masses have uniform temperature and humidity
characteristics
– They affect vast areas
• Fronts are boundaries between unlike air masses
– Fronts are spatially limited
– They are inherently linked to mid-latitude cyclones
Formation of Air Masses
– Source regions: The areas of globe where air
masses form are called source regions.
Surface conditions transfer to overlying air to
form air mass.
(1) air gains temperature and humidity
characteristics of the surface.
(2) Topographically uniform areas.
It will take a few days for an air mass to take on
the characteristics of the underlying surface. So,
air masses are usually formed in low and high latitude,
since middle latitude is too variable. It requires large
regions and uniform topography to form air masses.
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(3) It requires days for temp/moisture
imprinting to form air masses.
(4) air masses classified by temp/moisture
characteristics of source region
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moisture: continental (dry) v. maritime (marine) – c or m
temp: tropical (warm), polar (cold), arctic (very cold) – T, P or A
• Once formed, air masses migrate within the general circulation
• Upon movement, air masses displace residual air over locations thus
changing temperature and humidity characteristics
• Further, the air masses themselves
moderate from surface influences
North American air masses and
air mass source regions
Continental Polar (cP) and Continental Arctic (cA) Air Masses
Continental Polar (cP)
• Winter: originate over high-latitude land masses (northern Canada, Siberia)
• Low solar angle, short days, high albedo  cooling of over-lying air 
inversions and highly stable conditions
• very cold and dry  limited cloud formation, bright and sunny
• Summer: warmer with higher moisture content  fair weather cumulus
develop
Continental Arctic (cA)
• extremely cold and dry conditions: low temperature  limited vapor content
• boundary between cA and cP  Arctic Front
• Migrations of cP air  induce colder, drier
conditions over affected areas
• As air migrates heat and moisture capacity
increases  stability decreases
Maritime Polar (mP) Air Masses
• ~ to cP but warmer and higher moisture content
• forms over N. Pacific/Atlantic
• e.g. cP flowing out of Asia, over warm Japan current  adds
heat/moisture
• west coast of the US  mP air affects
regions during winter
• process is different along east Coast 
cyclonic flow: winds approach coast
from the northeast (nor’easters)
 cold, heavy snowfall
Continental Tropical (cT) Air Masses
• Summer phenomena – hot, low-latitude areas  SW US, northern Mexico
• little available moisture, high temp  hot, dry air masses
• steep lapse rates, unstable conditions due to intense surface heating 
limited cloud formation
• thunderstorms may occur:
• advection
• orographic lifting
Maritime Tropical (mT) Air Masses
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Form over low latitude oceans: Gulf of Mexico, tropical Atlantic
very warm, humid  unstable
affects southeastern US
Migration inland heating of air mass from ground surface increases
lapse rates  increases instability  intense ppt (thundershowers) .
• moisture content: reduced in northward
direction  Miami v. Chicago
Fronts
• separate air masses  leads to changes in temperature and humidity
as one air mass is replaced by another
• changes in temp  lead to uplift and ppt
• four types of fronts:
cold  cold advancing on warm
warm  warm advancing on cold
stationary  air masses not advancing
occluded  does not separate tropical
from polar/arctic, boundary btw two
polar air masses
Cold Fronts
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cold air displaces warm air
results in heavy precipitation events and rapid temperature drops
Extreme precipitation  extensive vertical lifting
warm air ahead of the front  forced aloft  cumulonimbus clouds
Frontal development about a
low pressure system
The vertical displacement of air along a cold front boundary
Types of fronts
Identification of fronts
1. Sharp temperature changes
2. Change in the air’s moisture
3. Shifts in wind direction
4. Pressure and pressure change
5. Clouds and precipitation
patterns
Course: Introduction to Atmospheric sciences(ATOC210) by GyuWon LEE
Consequence: cloud and precipitation patterns
Cold front
-Showers with gusty winds are prevalent when the warmer air is moist and
unstable
- Anvil with Ci (Cirrus), Cs (Cirrostratus).
-Move faster (15 ~25knots) and are steeply sloped (~ 1: 50)
-Ac: Altocumulus (mid-clouds); Cb: Cumulonimbus.
The sharp cold front boundary is evident
on both satellite pictures and radar composites
Consequence: cloud and precipitation patterns
Cold front: frontogenesis (frontolysis)
Warm Front
• occurs when warm air displaces colder air
• overrunning – warm air gently rides over cold, dense air
• leads to gradual progression of cloud types  stratus, nimbostratus, altostratus,
cirrostratus, cirrus
• cirrus is seen first, clouds continue to thicken and lower
• zone of contact is less steep than cold fronts  greater horizontal extent
• longer, less intense periods of rain (uplift not as dramatic)
• ppt falls through cold air mass  frontal fog, or sleet/freezing rain
Identification of fronts
Warm front
1. Sharp temperature changes
2. Change in the air’s moisture
3. Shifts in wind direction
4. Pressure and pressure change
5. Clouds and precipitation
patterns
- Advancing warm, moist,
subtropical (mT) air replaces
the retreating cold maritime
polar air.
- Precipitation well in advance
of the front’s surface boundary.
Consequence: cloud and precipitation patterns
Warm front
- Wide spread rain ahead of the sfc front (“overrunning”)
- Ci, Cs, As, Ns
- The wind veers with altitude
-Frontal inversion in the region of the upper level front
- Move slower (10knots) and are gently sloped (~ 1: 200)
Stationary Fronts
• contact zones sometimes are stalled  relatively ‘fixed’ in position
• relationship btw the air masses ~, do not move as quickly
• subjective in terms of speed  difficult to establish contact zone precisely
• Fronts may slowly migrate and warmer air is displaced above colder
Occluded Fronts
• Occluded front (occlusion): closure  cold air mass cuts off warm air
from ground
• separate cold/warm air masses BUT at surface air masses merge
• cold-type v. warm-type occlusion
– cold-type occlusion: eastern half of the continent  cP air meets
mP
– warm-type occlusion: western edges of continent  mP advances
on cP
Occlusion
sequence
Alternative Mechanisms: Occluded Fronts
Some occlusions form when the surface low
elongates and moves away from the junction of
the cold and warm fronts
Some occlusions occur when the intersection
of the cold and warm fronts slides along
the warm front
Consequence:
cloud and precipitation patterns
Cold-type occluded front
The faster moving cold front
catches up to the slower moving
warm front, and forces the warm
air mass to rise off the ground.
Consequence:
cloud and precipitation patterns
Warm-type occluded front
The faster moving cold
front overtakes the slower
moving warm front. The
lighter air behind the cold
front rises up and over the
denser air ahead of the
warm front.
Drylines
• Fronts based on temp and density differences of air
masses
• Humidity (moisture content) affects density  humid
air < dense (H20 lower molecular weight than N2 and
O2)
• Dryline: boundary btw humid and dry air.
A dryline over Texas