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Fronts
Fronts represent the interface between different air masses and are generally accompanied by
substantial changes in weather. The most common front in southern Australia is the cold front when
a colder air mass is displacing a warmer one (Figure 18.3). Cold fronts, which are usually linked to
low-pressure systems, occur in two forms:
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Type A: a deep, rapidly moving, cold wedge of air, lifting unstable warm air. As the front
approaches, warm humid northwestery winds freshen, carrying scattered low cloud. High clouds
increase and become darker and lower. The
‘anvil’ head often present with thunderstorm clouds may be visible. Light rain quickly turns to
heavy showers and thunderstorms with associated gusty winds. The winds turn cooler and west to
southerly as the front passes.
Type B: a shallow, slow moving wedge of cold air lifts moist, stable warm air. Because of the slow
movement and the shallowness of the wedge, the vertical motion of the warm air is relatively slow.
High clouds extend well ahead of this type of front. The frontal cloud extends back behind the front
for many kilometres and rain falls over a wide area.
The whole process of the Type B front passing a particular locality is slower than that of Type A. The
high cirrus clouds may precede a front by 12 to 36 hours.
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