AIR MASSES AND FRONTS

CP
20 TO 30 MPH
150 O
140 O
30 O
130 O
50
shown in figure 4-5. In the mid-latitudes, for an air mass
to be classified as arctic, the surface temperature is
generally 0 degrees Fahrenheit (-18 degrees Celsius) or
below.
TRAJECTORY PATHS A AND B
(CYCLONIC).—Paths A and B (fig. 4-5) are usually
indicative of a strong outbreak of cold air and surface
winds of 15 knots or more. This wind helps to decrease
140 O
O 50O
40 O
130 O
130 O
60 O
30 O 120 O
40 O
120 O
40O
110 O
60 O
50 O
100 O
40 O
4-8
100 O
60 O
40 O
90 O
70 O
60 O
50 O50O
G B
30 O
120 O
E
F
110 O
D
100 O
30 O30O
Figure 4-5.—Trajectories of cP and cA air in winter.
TURBULENCE
SNOW
FLURRIES
90 O
60 O
50 O
C
90 O
80O
90 O
A
30 O
80 O
70 O
the stable conditions in the lower levels. If this modified
air moves rapidly over rough terrain, the turbulence
results in low stratocumulus clouds and occasional
snow flurries (see fig. 4-6).
A particularly troublesome situation often arises
when the cold air flows from a cold, snow-covered
surface to a water surface and then over a cold,
snow-covered surface again. This frequently happens
with air crossing the Great Lakes. (See fig. 4-7.)
40 O
80 O
60 O
AG5f0405
30O
CLEAR AND COLD
N S
CP
O
TEMP. 2000' 10 F
TEMP. O
DIFFERENCE + 10 F
O
SURFACE TEMP. 0 F
NW
O
WATER TEMP. 34 F
MOISTURE AND
WARM
AIR RISING
Figure 4-6.—cP air moving southward.
SNOW FLURRIES
O
SURFACE TEMP. 21 F
Figure 4-7.—cP air moving over the Great Lakes (winter).
APPALACHIAN
MOUNTAINS
CLEAR
3 C/1000'
O
AG5f0406
O
10 F
SE
AG5f0407