Download ch9Understanding Weather_1

Ch 9: Air Masses and Fronts
Introduction
• Air masses are distinguished by 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
• Surface energy and moisture exchanges imprinted in
temperature, pressure, and moisture characteristics of
overlying air
• near surface conditions control in large part the condition
of atmosphere
• exchanges – vary through space and time
Source Regions
• areas in which air masses form  requires days for temp/moisture
imprinting
• high/low latitudes  require large regions of land or water and uniform
topography
• air takes on temperature and humidity characteristics of the surface below
• air masses classified by moisture and temperature characteristics of source
region
• moisture: continental (dry) v. maritime (marine) – c or m
• temp: tropical (warm), polar (cold), arctic (very cold) – T, P or A
•Thus, 5 catagories of air masses: cT, cP, cA, mT, and mP (no mA)
North American air masses and air mass source regions
cA
mP
mP
cP
Continental
Tropical
cT
mT
mT
mT
• Once formed, air masses migrate
• Upon movement, air masses displace residual air over locations they
overtake and change the temperature and humidity of those locations
• In addition, the air masses themselves
change from surface influences
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
cP
cT or mT
cA
• Migrations of cP air  induce colder, drier
conditions over affected areas
• As air migrates it warms and moisture capacity
increases  stability decreases
Maritime Polar (mP) Air Masses
• similar to cP but warmer and higher moisture content
• forms over N. Pacific/Atlantic
• Example: cP flows out of Asia, over warm Japan current  adds
heat/moisture and becomes mP
• Affects northwestern U.S. and Canada year round
• California coast of the US  mP air affects
regions mainly during winter
process is different along east Coast:

• cyclones cause winds to approach
coast from the northeast
(nor’easters)
• associated with cold and heavy
snowfall
Example of Nor’easter
Counterclockwise flow around east
coast low pressure brings cold
moist air to east coast from the
northeast
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
•However, limited cloud formation
due to low moisture content
• thunderstorms may occur:
• if unstable layer is deep
• if orographic lifting occurs
Continental
Tropical
cT
Maritime Tropical (mT) Air Masses
•
•
•
•
Form over low latitude oceans: Gulf of Mexico, tropical Atlantic, Pacific
very warm, humid  unstable
Huge influence on southeastern US
Migration inland heating of air mass from ground surface increases
lapse rates  increases instability  intense precip (thundershowers) 
localized uplift
• moisture content: reduced in northward
direction  Miami v. Chicago
• Arizona monsoon 
mT from Gulf of California in
late summer
Fronts
• separate air masses  movement of fronts leads to changes in temperature
and humidity as one air mass is replaced by another
• changes in temp  lead to uplift and precip
• 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 between two
polar air masses
Cold Fronts
•
•
•
•
cold air displaces warm air move at speeds of 0-50 km/hr
results in heavy, short-lived precipitation events and rapid temperature drops
Sometimes get extreme precipitation due to extensive vertical lifting
warm air ahead of the front is forced aloft  clouds form
Frontal development about a
low pressure system
The vertical displacement of air along a cold front boundary
The sharp cold front boundary brings narrow bands of short-lived precipitation
Warm Fronts
• occur 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
• From viewer’s perspective: cirrus is seen first, clouds continue to thicken and
become lower as warm front approaches (usually from south or southwest)
• zone of contact is less steep than cold fronts  greater horizontal extent
– longer, less intense periods of rain (uplift not as dramatic)
• precipitation falls through cold air mass  may getfrontal fog, or sleet/freezing rain
Warm frontal precipitation is less intense and spread over a broader
region than cold front precipitation
Stationary Fronts
• contact zones sometimes are stalled  relatively ‘fixed’ in position
• classification is 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  faster moving cold air mass
cuts off warm air from ground
• separate cold/warm air masses BUT at surface, cold 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
Drylines
• Fronts can be based on temp and/or density differences of air
masses
• Humidity (moisture content) affects density  humid air < dense
(H20 lower molecular weight than N2 and O2)
• Dryline: boundary between humid and dry air  Great Plains of
NA (spring/summer)  severe storms
• Dew points: east side (humid) is greater than west (dry air) 
 lead to tornadoes and severe thunderstorms
A dryline over Texas
Concepts
• Understand air masses
– How they are formed
– 5 main types
• Understand fronts
– 4 types and their characteristics
– Precipitation associated with warm and cold fronts
– drylines
Next Class:
• Hand back quizzes
• Read chapter 10 for lecture
• Lecture may cover severe weather too (no reading req’d)