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:
- 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.