Nature Of The Source Area

The basic idea of airmass formation is that radiative and turbulent transfers of energy and moisture, between the land or ocean surface and the atmosphere, give rise to

Frontal Zone Cross Section
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 airmasses are necessarily areas of extensive, uniform surface type that are overlaid by quasi-stationary pressure systems. These requirements are fulfilled where there is slow divergent flow from the major thermal and dynamic high-pressure cells. In contrast, low-pressure 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 hemisphere 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 9.2 Airmasses in winter. (A) Northern hemisphere. (B) Southern hemisphere.

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.

Figure 9.4 Airmasses in summer. (A) Northern hemisphere. (B) Southern hemisphere.

Sources: (A) After Petterssen (1950) and Crowe (1965). (B) After Taljaard et a/. (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.

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