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Fundamentals of
Physical Geography 1e
Chapter 6: Air Masses and Weather Systems
Petersen
Sack
Gabler
Air Masses and Weather Systems
Air Masses
• Air Mass: large body of air whose temperature and
moisture characteristics are similar
• Source Region:
m = Maritime (sea)
c = Continental (land)
E = Equatorial (very warm)
T = Tropical (warm)
P = Polar (cold)
A = Arctic (very cold)
Air Masses
•Air Masses
– Maritime Equatorial (mE)
– Maritime Tropical (mT)
– Continental Tropical (cT)
– Continental Polar (cP)
– Maritime Polar (mP)
– Continental Arctic (cA)
Air Masses
Air Masses
• Air Mass Modification
and Stability
– cP or cA moves over
Great Lakes and picks
up moisture, and may
cause Lake effect
snow.
What two main factors
contribute to increased
precipitation caused by the
lake effect?
Air Masses
• North American Air
Masses
– Continental Arctic (cA)
– Continental Polar (cP)
– Maritime Polar (mP)
– Maritime Tropical (mT)
– Continental Tropical (cT)
Air Masses
• Continental Arctic (cA): Dry and very cold
– Occasionally impacts U.S. in the winter
– Often produces record breaking cold
• Continental Polar (cP): Dry and cold
– May reach Gulf of Mexico in winter
– Rarely affects west coast
• Maritime Polar (mP): Moist and cool
– Westerlies bring air mass to west coast, especially
in winter
– Occasionally affects eastern U.S. as Nor’easters
Air Masses
• Maritime Tropical (mT): moist and warm
– May originates in Gulf of Mexico
– Major impact on central and eastern U.S
•T-storms in the summer
•Clashes with cP
• Continental Tropical (cT): dry and hot
– Small source region (SW deserts and N. Mexico
– Dry line
– Smallest player in U.S. weather
Air Masses
Which air masses affect your location? Are there
seasonal variations?
Fronts
• Fronts
– Clash between air masses
– Sloping boundary
– Generally move with westerlies
– 3-dimensional
– Frontal uplift
– U.S. and Canada in a zone between source
regions
Fronts
• Cold Front:
– Cold air moves in on
warm air
– Warm air (less dense)
rises above cold air
– Steep slope
– Cumulonimbus
– May form a Squall line
– Sharp changes in
temperature, pressure,
and wind
Cirrus
anvil
top
Warm air
mass
Cumulonimbus
Cold air
mass
Cumulus
Stepped Art
Fig. 6-3, p. 127
Fronts
• Warm Front:
– Warm air moves in on
cooler air
– Warm air (less dense)
rises above cold air
– Slope is not as steep
– Light precipitation
which may last longer
– Usher in warmer
conditions
Warm air mass
Cirrus
Cirrostratus
Altostratus
Cool air mass
Nimbostratus
Stratus
Stepped Art
Fig. 6-4, p. 128
Fronts
• Stationary Front
– Boundary between air masses that is not moving
– Extended period of light precip. and occasionally
strong T-storms
• Occluded Front
– Cold air is overtaking warm air
– Dying storm
Four major frontal symbols used on weather maps
Atmospheric Disturbances
• Anticyclones and
Cyclones
– Atmospheric disturbance
•Anticyclone
•Cyclone
•Wind and pressure
gradient
Where would be the strongest
winds in this figure? Where
would be the weakest winds?
Atmospheric Disturbances
• Anticyclone (H)
– Move with path of westerlies
– Divergence, sinking air
– Sources:
•Northern Canada and Arctic  Polar outbreak
•Subtropical High  Dry and warmer weather
Atmospheric Disturbances
• Cyclones
– Low pressure
– Convergence and rising air
– Clouds and precipitation
Atmospheric Disturbances
• Mapping pressure systems
– Horizontal structure
– Vertical Structure
Atmospheric Disturbances
• General Movement
– Track of storms (mid-latitude cyclones)
– What storm track influences your location?
Atmospheric Disturbances
• Middle-Latitude Cyclone
– Also known as extratropical cyclones
– Migrating storms
– Clash between air masses
•Ex: Cp vs. mT
– Vary in intensity, longevity, speed of travel, wind
strength, amount and type of cloud cover, the
quantity and type of precipitation, and the area
they affect.
– Associated with polar front
•Movement with the seasons
Atmospheric Disturbances
• Middle-Latitude Cyclone
– Stages in the development of a mid-latitude cyclone
– In (c), where would you expect rain to develop. Why?
Atmospheric Disturbances
• Cyclones and
Local Weather
Describe the
front(s), temp,
air mass type,
wind direction,
and precip. in
Pittsburg and
Detroit.
Atmospheric Disturbances
• Cyclones and the Upper
Air Flow
– Alternating pressure
ridges (highs/divergent
winds) and troughs
(lows/convergent winds)
Where would you
expect storms to
develop?
Atmospheric Disturbances
• Cyclones and the Upper Air Flow
– Polar Jet Stream Analysis
– Which country does most of this pattern occupy? Where is
one trough?
Atmospheric Disturbances
• Hurricanes
– Circular, cyclonic storm with wind
speeds greater than 74 mph
– Smaller than mid-latitude cyclone
– Same air mass type
– More destructive
– Calm winds at center
– Also called tropical cyclones
– Require/fueled by warm water
(80oF or more)
Atmospheric Disturbances
• Hurricane cross-section
Atmospheric Disturbances
• Major “Hurricane Alleys”
Which coastlines seem unaffected by these tracks?
Atmospheric Disturbances
• Hurricane development
– Warm water (>27oC or 80oF)
– Most air
– Coriolis force (does not form or survive near equator)
• Stages of development
– Tropical disturbance (easterly waves)
– Tropical depression
– Tropical storm (becomes named; 39-74 mph)
– Hurricane (categorized by Saffir-Simpson Scale)
– Dissipation (dies) over land or cool water
Atmospheric Disturbances
• Hurricane Intensities and Impacts
– Storm surges
– Saffir-Simpson Scale
What can people who live in such regions do to protect
themselves when a serious storm surge is threatening?
Atmospheric Disturbances
• Saffir-Simpson Hurricane Scale
Atmospheric Disturbances
• Hurricane Intensities and Impacts
– 2004 was a record- breaking year
• 3 in Florida
– Hurricane Katrina in 2005
• New Orleans
• Levee failure
– Hurricane Ike in 2008
Atmospheric Disturbances
• Snow storms and
blizzards
– Mid and high latitudes
• Blizzard
– Severe weather event
– Heavy snow and
strong winds (35 mph)
– Visibility reduced
Atmospheric Disturbances
• Thunderstorms
– Low and mid latitudes
– Lightning: intense
discharge of electricity
– Thunder: sonic boom
created by the
expansion of air
around the lightning
bolt
Atmospheric Disturbances
• Types of Thunderstorms
– Convective (thermal)
– Orographic
– Frontal
Atmospheric Disturbances
• Tornadoes
– Occur almost
anywhere but are
most common in
North America
(Tornado Alley)
– Small intense,
cyclonic storm of
low pressure,
violent winds, and
converging air
Destruction caused by an F5 tornado in Greensburg, Kansas on May 16, 2007
Atmospheric Disturbances
• Tornado
– Typically small and
short lived
– 80% associated
with thunderstorms
– Temporal Variability
•March to July
•Late afternoon
or early evening
Atmospheric Disturbances
• Doppler radar:
– Improves tornado
detection and
forecasting
– Able to determine
wind speed and
direction
– Hook Echo is a
signature of a
tornado
Atmospheric Disturbances
• Fujita Scale (F-0 to F-5)
• Enhanced Fujita Scale (EF-0 to EF-5)
Weather Forecasting
• Weather Forecasting
– Doppler radar
– Weather satellites (e.g.
GOES East)
– High speed computers
– Improving!
Is there cloud cover
over your state on this
day?
Fundamentals of
Physical Geography 1e
End of Chapter 6:
Air Masses and Weather Systems
Petersen
Sack
Gabler