The concept of 'fronts' arose after the 1914-18 war from the similarity of the interface between different air masses to the battle-front between opposing armies in France. The idea was introduced by a handful of meteorologists working in a spare room of the home of Vilhelm Bjerknes in Bergen (Norway). He developed the idea in co-operation with his son, Jacob, and Tor Bergeron, Carl-Gustav Rossby and others, who became known as the 'Bergen School'. No group in history has had a larger impact on the way we think about weather today.
It can be seen in Figure 13.1 that adjacent winds blow in contrary directions over the southeast of Australia; to the left, an mP air mass is advected from the south-west, and, to the right, much warmer mT air enters from the north. So a line with triangular protuberances has been drawn on the map at the boundary between the two air masses (Chapter 15). This line represents a cold front, advancing in the direction of the triangles.
Criteria for a front vary, but typically a meteorologist will look for a horizontal temperature gradient of at least 3 K/100 km in subtropical regions (more at higher latitudes), and a difference of wind direction by at least 60°. The whole pattern of temperatures, winds, uplift, humidity, clouds and precipitation around a front constitutes a frontal system, including a frontal (transition) zone, where the differences of temperature etc. are most abrupt. The frontal boundary is a 'cold front', when a cold air mass moves into a warmer area. A warm front arises when a warm air mass advances into a colder region, repelling and riding over the cooler air mass. Conventional symbols are shown in Figure 13.2.
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