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 SymbolArctic APolar PTropical TEquatorial EAntarctica 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 Symbolcontinental arctic cAcontinental polar cPcontinental tropical cTmarine tropical mTmarine polar mPmarine Equatorial mEcontinental Antarctic 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 highArctic (cA)

*Distinguished by relative difference in temperaturecP-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 lake-effect 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