Chapter 8

Air MassesThe 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

Movementof a

Cold and DryAir Mass

BringsWinter 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 MassSource Regions

Classifying Air Masses

Temperature (latitude)P-PolarA-ArcticT-Tropical

Humiditym-Maritime-wet (generally over water)c-Continental-dry (generally over land)

A Maritime Air Mass Source-region

Air Mass Modification

k-coldw-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

ExtraordinaryLake-EffectSnowstorm

AtmosphericHazard

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.

Images: NOAA, USCG

Nor’easters

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