Geoscience Reference
In-Depth Information
Figure 9.3
The average vertical
temperature structure for selected
airmasses affecting North America at
about 45 to 50°N, recorded over their
source areas or over North America
in winter.
Sources
: After Godson (1950), Showalter
(1939), and Willett.
Figure 9.4
Airmasses in summer. (A) Northern hemisphere. (B) Southern hemisphere.
Sources
: (A) After Petterssen (1950) and Crowe (1965). (B) After Taljaard
et al
. (1969) and Newton (1972), by permission of the American
Meteorological Society.
temperature inversion from the surface up to about 850
mb in typical cA or cP air. Because of their extreme
dryness, small cloud amounts and low temperatures
characterize these airmasses. In summer, continental
heating over northern Canada and Siberia causes the
virtual disappearance of their sources of cold air. The
Arctic Basin source remains (Figure 9.4A), but the cold
air here is very limited in depth at this time of year.
In the southern hemisphere, the Antarctic continent and
the ice shelves are a source of cA air in all seasons
(see Figures 9.2B and 9.4B). There are no sources of cP
air, however, due to the dominance of ocean areas in
middle latitudes. At all seasons, cA or cP air is greatly
modified by a passage over the ocean. Secondary types
of airmass are produced by such means and these will
be considered below.
