Air Masses

Large bodies of air.
>1600 km across; several km thick.
Nearly homogenous physical properties, especially temperature and pressure.
Forms in a Source Region.
May take several days to move from source region through adjacent areas.
Nearly constant weather condition prevail within air mass: Air-Mass Weather.

Source Region

Areas in which the air mass originates.
Gives the air mass its characteristics of. temperature and pressure.
Region must be large and physically uniform.
Generally, these areas have significant periods of stagnant air movement.
This allows time for air mass to acquire its characteristics
Typically, areas dominated by slow moving anticyclones (high pressure)

Classification of Air Masses

Based on 2 aspects

1) Latitude:
3 (or 5) class system
A (Arctic): highest latitudes, coldest air masses
P (Polar): Mid- to high latitudes, cold air masses
T (Tropical): Low latitudes, warm air masses

2) Source Area Surface:
m (maritime): oceans, seas or gulfs
Generally (but not always) results in moist, humid air masses.
c (continental): large land masses
Generally results in dry, arid air masses.

Two letter scheme, combining:
small case Source designation
upper case Latitude designation.
Possible combinations:
mA - maritime Arctic:
cA - continental Arctic:
mP - maritime Polar:
cP - continental Polar:
mT - maritime Tropical:
cT - continental Tropical:

Modification to the standard system
The lower case letter "w" is added to the air mass classification to indicate that the air mass is warmer than the surface over which it is passing.
"k" is added when the air mass is colder.

North American Air Masses

Principle air masses affecting North America:
Each air mass can be characterized by:

mA - maritime Arctic
Source Region:
Temperature/Humidity Conditions:
Stability:
Associated Weather:

cP - continental Polar
Source Region:
Temperature/Humidity Conditions:
Stability:
Associated Weather:

mP - maritime Polar
Source Region:
Temperature/Humidity:
Stability:
Associated Weather:

mP - maritime Polar
Source Region:
Temperature/Humidity Conditions:
Stability:
Associated Weather:

cT - continental Tropical
Source Region:
Temperature/Humidity Conditions:
Stability:
Associated Weather:

mT - maritime Tropical
Source Region:
Temperature/Humidity:
Stability:
Associated Weather:

mT - maritime Tropical
Source Region:
Temperature/Humidity Conditions:
Stability:
Associated Weather:

Lake Effect Snow

Occurs downwind of large inland lakes, such as the Great Lakes
Cold air moves over a warm body of water.
Moisture content of the air is increased.
Temperature of the air warms slightly - causing lifting.
Air mass rise causing cooling, condensation and precipitation.

Siberian Express

Outbreaks of very, very cold air masses from the Siberian High which develops in winter.
Very cold air moves over the north pole and descends into North America.
Can reach the southern tier states, and even extend into Central America.

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