About a hundred cold fronts track along the southern coasts of South Africa and Australia annually, i.e. about two a week on average. Most derive from low pressures at about 60°S, extending into the troughs between subtropical highs (Figure 12.1 and Figure 12.7), as in Figure 13.1 for a particular day. They are typically oriented north-west—south-east and tend to
Figure 13.5 The correspondence of cloud seen from a satellite and the position of a front, both at noon GMT on 8 November 1995. The dashed line indicates the position of a prefrontal trough (Figure 13.2).
behind the southern end. The northern end may extend well towards the equator, especially in winter. For instance, there is a cold front through Mt Isa (at 20°S in Queensland) several times a year in the form of a shallow wedge of cooler air, travelling as a density current (Note8.C ) through the PBL. Ripples form at night as the front ploughs through the stable air, and they can run ahead on top of the PBL to create a Morning Glory in the Gulf of Carpentaria (Note 8.L).
Cold fronts in South America can travel as far as 5°S in the lee of the Andes (Figure 13.8). Such an incursion of polar air, known as a friagem, can greatly harm Brazil's coffee crop (Section 3.6).
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