Geoscience Reference
In-Depth Information
regions are zones of convergence into which airmasses
move (see F, this chapter).
The major cold and warm airmasses will now be
discussed.
1 Cold airmasses
The principal sources of cold air in the northern hemi-
sphere are (1) the continental anticyclones of Siberia
and northern Canada where continental polar (cP)
airmasses form, and (2) the Arctic Basin, when it is
dominated by high pressure in winter and spring (Figure
9.2A). Sometimes Arctic Basin air is designated as
continental Arctic (cA), but the differences between cP
and cA airmasses are limited mainly to the middle and
upper troposphere, where temperatures are lower in the
cA air.
The snow-covered source regions of these two
airmasses lead to marked cooling of the lower layers
(Figure 9.3). Since the vapour content of cold air is very
limited, the airmasses generally have a mixing ratio
of only 0.1-0.5 g/kg near the surface. The stability
produced by the effect of surface cooling prevents
vertical mixing, so further cooling occurs more slowly
by radiation losses only. The effect of this radiative
cooling and the tendency for airmass subsidence in
high-pressure regions combine to produce a strong
Figure 9.1
A schematic height cross-section for the northern
hemisphere showing barotropic airmasses and a baroclinic frontal
zone (assuming that density decreases with height only).
distinctive physical properties of the overlying air
through vertical mixing. A degree of equilibrium
between the surface conditions and the properties of the
overlying airmass will be achieved if the air remains
over a given geographical region for a period of about
three to seven days. The chief source regions of air-
masses are necessarily areas of extensive, uniform
surface type that are overlaid by quasi-stationary pres-
sure systems. These requirements are fulfilled where
there is slow divergent flow from the major thermal and
dynamic high-pressure cells. In contrast, low-pressure
Figure 9.2
Airmasses in winter. (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.
