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
(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
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.)
Figure 4-5.Trajectories of cP and cA air in winter.
20 TO 30 MPH
CLEAR AND COLD
Figure 4-6.cP air moving southward.
DIFFERENCE + 10 F
2000' 10 F
SURFACE TEMP. 0 F
WATER TEMP. 34 F
SURFACE TEMP. 21 F
Figure 4-7.cP air moving over the Great Lakes (winter).