Concept of Air Mass :
* The air over a very large region with similar & persistent properties near the surface :
e.g. wind, temperature and moisture content .
* An air mass forms over a source region
* A shallow surface layer of air
in contact with a large land mass or
a large oceanic region for a long time
would be dictated by the characteristics of the surface as a result of prolonged :
# radiational cooling/heating,
# heat exchange with the surface,
# evaporation from the surface, and
# thorough mixing within the surface layer.
* That would happen if there is little lateral mixing with air over the neighboring regions
Characteristics of a source region for an air mass :
* flat & relatively homogeneous surface;
e.g. arctic plain , desert, subtropical oceans
* light wind;
Major factors that determine the characteristics of an air mass :
(1) latitude
(2) land vs ocean
(3) Season
(4) surface condition
4 general categories
(1) Polar (P) air mass ; (A) arctic ; cold
(2) Tropical (T) air mass , hot
(3) Continental (c) air mass , dry
(4) Maritime (m) air mass, moist
Movement of air mass
* An air mass could occassionally become unstable &
move out of its source region,
or move under the influence of the large scale flow in the upper levels,
* soon it would lose its identity.
e.g. when cP air mass moves to an oceanic area;
quickly becoming warmer and more moist ;
* when cP air moves over the Great Lakes, it may induce heavy lake-effect snow over the eastern shore .
Air masses that affect N America (Fig.8.2) :
* Presence of an air mass gives rise to persistent weather
* Example 1 :
mT air from the Gulf of Mexico brings sultry conditions in south-eastern US during summer &
repeatedly gives rise to afternoon thunderstorms
* Example 2 :
mP air from Pacific brings widely scattered showers to northwestern US
for an extended period of time in winter
Types of influences
(1) cP or cA from northern Canada & Alaska in winter could become so cold &
the pressure would be so high,
that the flow would become unstable.
* It would surge southward ;
mostly influence the mid-west and the east coast;
west coast is shielded by the Rockies;
e.g. Cold Outbreak over central US (ex. Dec/83, Fig.8.3, p192)
referred to as "Siberian Express", or "Alberta Clipper"
a cold wave with high pressure as it plunges south-eastward
snow storm; then clear sky, extremely cold & fair sky
(2) Pacific mP air mass, from N Pacific (Fig.8.5)
* mostly influences west coast
* air at ~ 1 km above may be of the cP type, much colder freezing
* as surface air moves up the western slope of the Rockies,
condensation occurs, turns into snow
* eastward journey changes the air mass to cP
(3) Atlantic mP air mass over cold water in Labrador current
* influences northeast of US,
* damp cold air,
* low and thick cloud,
* rain, drizzle or snow.
* called New England nor'easter
(4) mT air mass from subtropical east Pacific Ocean ,
* very warm and moist,
* precip along west coast & northwest.
(5) mT air mass from Gulf of Mexico & Carribean Sea (Fig.8.8)
* except for winter, strongly affects all areas east of the Rockies
* persistent weather conditions in the southern states
* afternoon showers and thunderstorms along the Gulf coast in summer
* A disturbance over the central plain in winter may advect the Gulf mT air mass eastward,
resulting an unseasonally warm episode along the east coast.
(6) cT air mass in summer from northern Mexico and southwestern US (Fig.8.10, p199)
* hot , dry , unstable at low levels ;
* as it invades the high plain region, there would be an upper level ridge ;
jet stream would be displaced northward ;
weak subsidence,
surface air even warmer by continual solar heating (Fig.8.10)
Heat Wave
Boundary between two air masses in the presence of an active weather disturbance would
* intensify temperature/moisture contrast;
* Produce atmospheric fronts ;
* lead to severe storm in spring & blizzard in winter