Download Weather & Climate Chapter 8

CHAPTER 8
Air Masses
Air mass
- Large mass of air characterized by:
(1) common properties of
temperature and humidity
(2) characteristics of their region of
origin source region
- Air mass is important concept in
meteorology
Source Region
- Term for a region whose conditions
create air masses
- Ideally it should meet the criteria of:
(1)Extensive and uniform in area
(2) Area of air stagnation
Air mass source regions are found in
high and low latitude regions
Source Region, cont
- They will migrate to the mid-latitudes
- They make the mid-latitudes a
conflict zone
- Aside: This idea of a conflict zone
became an important aspect
of the writings of Ellsworth
Huntington and the paradigm
of environmental determinism
Source Region, cont
- Air masses are classified by:
(1) latitude of the source region
Latitude
Symbol
Arctic
A
Polar
P
Tropical
T
Equatorial
E
Antarctica
AA
* differences are relative
Source Region, cont
- Air masses are classified by:
(2) surface-type of the source region
Surface Type
Symbol
continental
c
marine
m
Air Mass
- Taken Together:
Air Mass
continental arctic
continental polar
continental tropical
marine tropical
marine polar
marine Equatorial
continental Antarctic
Symbol
cA
cP
cT
mT
mP
mE
cAA
Air Mass, cont
-
In addition, we frequently add a
3rd factor (3) tendency
--- if air mass is colder than the surface it
passes over, a “k” is added after its
initial symbols (cAk – continental arctic
air colder than surface passing over)
--- if air mass is warmer than surface
passed over, a “w” is added (mTw –
marine tropical air warmer than surface
passed over)
Air Mass, cont
- “k” and “w” are indicators of stability
tendency
--- “k” situation leads to
instability
--- “w” situation leads to stability
- The air mass both modifies and is
modified by the surface it passes
over after leaving source region
Air Mass, cont
- Air mass influence on North
America
--- cP and cA
--- mP
--- mT
--- cT
Air Mass, cont
cA and cP
--- dry and cold
--- originate over central Canada
(cP) and Greenland and the high
Arctic (cA)
*Distinguished by relative difference in
temperature
cP-very cold cA-bitter
-
Air Mass, cont
cA and cP
- Greatest effect during N.A. winter (land&air
very cold)
… may bring brief cool/clear summer
weather
- Invades the central U.S. ; no mountains
- When crossing open Great Lakes will bring
precipitation and in winter produces lakeeffect snows
Air Mass, cont
mT
- Originate Caribbean and Gulf of Mexico,
lesser so tropical Pacific
… Pacific air has little effect N.A., though it
can bring extensive July-Aug
precipitation of SW
- Warm to hot. Humid and unstable
- Dominant summer air mass of central U.S.
- Potential for extensive precipitation and
violent weather patterns
Air Mass, cont
mP
- Forms over high latitude oceans
… may have originated as cP air over
Siberia (modified over Pacific)
- Cool to cold; humid and unstable compared
to cP
and cA
- Because of westerly air flow, Pacific mP
more important to N.A. than Atlantic
- Potential for heavy orographic effect
Air Mass, cont
cT
- N.A. doesn’t have the land mass form
extensive cT air
- However, Mexico and the desert SW
U.S, produce the hot, dry air
- Air is unstable, but unlike other
tropical air masses, cT is dry unstable
- Generally confined to source region,
may break out during very hot, dry
seasons – brings drought the Great
Plains