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Chapter 8
Air Masses
The Atmosphere 10e
Lutgens & Tarbuck
Air Masses
• Immense bodies of air
• ~1600 kilometers (1000 miles) across on
average, can be much wider
• Several kilometers thick
• Similar properties at a given altitude, such
as temperature or moisture content
Movement
of a
Cold and Dry
Air Mass
Brings
Winter Weather
The Siberian Express
Source Regions
• The areas where air masses originate
• The location of the source region largely
determines the characteristics of the air
mass
• A source region should have a relatively
uniform topography, and…
• Should have relatively stagnant atmospheric
motion for a period of time
A Hot and Dry Air Mass Source-region
Air Mass
Source
Regions
Classifying Air Masses
Temperature (latitude)
P-Polar
A-Arctic
T-Tropical
Humidity
m-Maritime-wet (generally over water)
c-Continental-dry (generally over land)
A Maritime Air Mass Source-region
Air Mass Modification
k-cold
w-warm
•As airmasses move away from source regions, they
change slowly
•May get warmer, cooler, more or less humid
•cP or cA airmasses that move over water in winter
can convert to mP airmasses
•If the airmass is colder than the surface it passes over,
we add a k; we add a w in the reverse
•Consider the stability implications
More Modification
• We just discussed temperature modification,
with some associated humidity aspects
• Basically associated with temperature
differences between airmass and ground
below
• There are also mechanical modifications
caused by airflow around weather systems
and/or topography such as mountains
cP and cA
• Air masses formed north of about 50°N
latitude
• cA is a little colder than cP, but many
meteorologists do not differentiate between
them
• Generally very dry, except when passing
over warmer bodies of water
• The cause of our cold snaps in fall and
winter
More cP and cA
• The source region for these air masses is
stable
• Can be modified to cPk in winter due to
passage over the Great Lakes
• This can result in “Lake Effect Snow” on
the lee side of the lakes (side opposite wind
flow)
Lake Effect
Snow
Atmospheric
Hazard
Extraordinary
Lake-Effect
Snowstorm
cP
Records from cP air masses
• Fastest temperature drop
– On January 23-24, 1916, temps dropped 55.5°C
(100°F) in 24 hours at Browning, Montana
• Most lake effect snow in Buffalo, NY
– Between December 24, 2001 and January 1,
2002, the city received 207.3 cm (81.6 in) of
snow
mP air masses
• Forms over oceans at high latitudes
• Cool to cold
• mP can come from
– The North Pacific
• Source region is mild (cool) and humid all year
• Source is unstable in winter, stable in summer
– The Northwestern Atlantic
• Source region is cold and humid in winter, cool and
humid in summer
• Source is unstable in winter, stable in summer
More mP
• mP from the North Pacific brings showers
in winter, heavy orographic (caused by
mountains) precipitation in winter, and low
stratus/fog in winter
• mP from the NW Atlantic brings occasional
“nor’easters” in winter (giant mid-latitude
cyclones along the east coast), occasional
periods of clear, cool weather in summer
Pacific mP air masses
• Control more of our weather than NW
Atlantic mP
• Usually begin as cP in Siberia
• Travel across the Pacific where they play a
large role in the weather experienced along
the West Coast of the U.S.
• Become more mild as they move across the
Pacific
Modification of cP Air Mass
NW Atlantic mP
• Only occasionally affect the U.S.
• Usually associated with “nor’easters” which
act to funnel the airmass back behind a low
pressure system into the U.S.
• “Nor’easters” are a wintertime event with
strong winds, cold to freezing temps, and
abundant precipitation
• In the summer, they bring pleasant cool air
as a break from the typical hot, humid
weather in the northeast U.S.
Nor’easters
Images: NOAA, USCG
mT air masses
• mT air masses originate in the GOM,
Caribbean, western Atlantic
• mT air masses also originate in the
subtropical Pacific
• While both source regions are warm and
humid all year, the Atlantic side is unstable
all year with the reverse occurring in the
Pacific side
More mT
• mT Atlantic can move north in the winter,
bringing precip and fog
• It dominates the southern U.S. in summer
with hot and humid conditions, as well as
plenty of rain
• mT Pacific occasionally brings drizzle and
light rain to the SW U.S. in winter
• It sometimes reaches the west U.S. in
summer, causing some thunderstorms
mT Air Moves over Heated Land
Resulting in
Cumulus Development and Showers
ISOHYETS
Desert Rainfall from mT Air Mass
cT airmasses
• Sometimes in summer, cT air forms in
Mexico and the interior southwestern U.S.
• The air is, as expected, quite hot, but there
is very little humidity
• Therefore the summer weather is generally
hot and dry, with large daily temperature
changes
• Rarely, cT air masses move north into the
Great Plains
Chapter 8
END