anticyclones have a divergent airflow that spreads the
properties horizontally over a large area; turbulence and
convection distribute these properties vertically.
Subsidence (downward motion), another property of
anticyclones, is favorable for lateral mixing, which
results in horizontal or layer homogeneity.
Warm highs, such as the Bermuda and Pacific
highs, extend to great heights because of a lesser
density gradient aloft and thereby produce an air mass
of relatively great vertical extent. Cold highs, such as
the Siberian high, are of moderate or shallow vertical
extent and produce air masses of moderate or shallow
height.
2.
Cyclonic systems. Cyclonic systems are not
conducive to air mass formation because they are
characterized by greater wind speeds than anticyclonic
systems. These wind speeds prevent cyclonic systems
from stabilizing. An exception is the stationary heat
low.
3.
Belts of convergence. Belts of convergence are
normally not conducive to air mass formation since
they have essentially the same properties as cyclonic
systems. However, there are two areas of convergence
where air masses do form. These are the areas over the
north Pacific, between Siberia and North America, and
the
Atlantic,
off
the
coast
of
Labrador
and
Newfoundland. These two areas act as source regions
for maritime polar air.
AIR MASS SOURCE REGIONS
The ideal condition for the production of an air
mass is the stagnation of air over a uniform surface
(water, land, or ice cap) of uniform temperature and
humidity. The length of time an air mass stagnates over
its source region depends upon the surrounding
pressures. From the surface up through the upper levels,
such air acquires definite properties and characteristics.
The resulting air mass becomes virtually homogeneous
throughout, and its properties become uniform at each
level. In the middle latitudes, the land and sea areas
with the associated steep latitudinal temperature
gradient are generally not homogeneous enough for
source regions. These areas act as transitional zones for
air masses after they have left their source regions.
The source regions for the worlds air masses are
shown in figure 4-1. Note the uniformity of the
underlying surfaces; also note the relatively uniform
climatic conditions in the various source regions, such
as the southern North Atlantic and Pacific Oceans for
maritime tropical air and the deep interiors of North
America and Asia for continental polar air.
4-2
SIBERIAN
SOURCE
CP
SOURCE
MP
SOURCE
MT
SOURCE
MT
SOURCE
CP
SOURCE
MT
SOURCE
MP
SOURCE
CP
SOURCE
NUMBER
CT
SOURCE
CT
CT
MT
SOURCE
MT
SOURCE
MT
SOURCE
MP
SOURCE
MP
SOURCE
MT
SOURCE
MT
SOURCE
MP
CT
60
40
20
0
20
40
100
120
140
160
160
180
140
120
100
80
60
40
20
0
20
40
60
80
100
AG5f0401
ARCTIC AIR
MASSES
A
WINTER
EQUATORIAL AIR
CT
MA
MT
Figure 4-1.Air mass source regions.