Warm airmasses

These have their origins in the subtropical high-pressure cells and, during the summer season, in the bodies of warm surface air that characterize the heart of large land areas.

The tropical (T) sources are: (1) maritime (mT), originating in the oceanic subtropical high-pressure cells; (2) continental (cT), either originating from the continental parts of these subtropical cells (e.g. as does the North African Harmattan); or (3) associated with regions of generally light variable winds, assisted by upper tropospheric subsidence, over the major continents in summer (e.g. Central Asia). In the southern hemisphere, the source area of mT air covers about half of the hemisphere. There is no significant temperature gradient between the equator and the oceanic subtropical convergence at about 40°S.

The mT type is characterized by high temperatures (accentuated by the warming due to subsidence), high humidity of the lower layers over the oceans, and stable stratification. Since the air is warm and moist near the surface, stratiform cloud commonly develops as the air moves poleward from its source. The continental type in winter is restricted mainly to North Africa (see Figure 9.2), where it is a warm, dry and stable airmass. In summer, warming of the lower layers by the heated land generates a steep lapse rate, but despite its instability the low moisture content prevents the development of cloud and precipitation. In the southern hemisphere, cT air is rather more prevalent in winter over the subtropical continents, except for South America. In summer, much of southern Africa and northern Australia is affected by mT air, while there is a small source of cT air over Argentina (see Figure 9.4B).

The characteristics of the primary airmasses are illustrated in Figures 9.3 and 9.5. In some cases, movement away from the source region has considerably affected their properties, and this question is discussed below (see p. 181).

Source regions can also be defined from analysis of airstreams. Streamlines of the mean resultant winds (see Note 1) in individual months may be used to analyse areas of divergence representing airmass source regions, downstream airflow and the confluence zones between different airstreams. Figure 9.6A shows airmass dominance in the northern hemisphere in terms of annual duration. Four sources are indicated: the subtropical North Pacific and North Atlantic anticyclones, and their southern hemisphere counterparts. For the entire year, air from these sources covers at least 25 per cent of the northern hemisphere; for six months of each year they affect almost three-fifths of the hemisphere. In the ocean-dominated southern hemisphere,

Figure 9.5 The average vertical temperature structure for selected airmasses affecting North America in summer.

Sources: After Godson (1950), Showalter (1939), and Willett.

Figure 9.5 The average vertical temperature structure for selected airmasses affecting North America in summer.

Sources: After Godson (1950), Showalter (1939), and Willett.

Figure 9.6 Airmass source regions. (A) Northern hemisphere. (B) Southern hemisphere. Numbers show the areas affected by each airmass in months per year.

Sources: After Wendland and Bryson (1981), and Wendland and McDonald (1986), by permission of the American Meteorological Society.

the airstream climatology is much simpler (Figure 9.6B). Source areas are the oceanic subtropical anticyclones. Antarctica is the major continental source, with another mainly in winter over Australia.

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