Introducing Meteorology by Jon Shonk

Author:Jon Shonk
Language: eng
Format: epub, mobi, pdf
Publisher: Dunedin Academic Press Ltd

Figure 10.3 The origins of air masses that affect the weather in (A) North America and (B) Europe.

10.3Low-Pressure Systems

The meeting of these contrasting air masses at the polar front leads to the development of mid-latitude weather systems. The generation of depressions – a process known as cyclogenesis – has been studied for a long time. The early work of Jakob Bjerknes and his group of scientists in Norway in the 1910s and 1920s (see Chapter 2) generated the so-called Norwegian model, which states that depressions form from waves that develop on the polar front by the steps shown in Figure 10.4. In its unperturbed state, the polar front is a long trough of low pressure with very strong wind shear across it – the warm air moves in a westerly direction and the cold air moves in an easterly direction. This makes the area very susceptible to rotation. In other words, if a region of stronger ascent (lower pressure) develops on the polar front, the air rushing in to the developing low-pressure centre will quickly begin to rotate. A kink develops in the polar front, centred on the pressure minimum. The developing kink is referred to as a frontal wave. As the system develops, the kink in the polar front continues to grow. The central pressure continues to fall, leading to sharper pressure gradients, stronger winds and more intense rain. At this stage, the system has the familiar structure of a warm front being chased by a cold front. At the warm front, warm air rides up over cold air; at the cold front, cold air undercuts warm air. Ascending air at both fronts continues to form cloud and rain along their lengths, with most of the rain falling towards the centre.

The depression then begins to decay. The first sign of the depression’s demise is the process of occlusion. The fronts continue to spiral around each other until the cold front starts to undercut the warm front, lifting it up off the ground, resulting in an occluded front. The depression now has three fronts associated with it, all meeting at the triple point. The winds in the system start to weaken, although cloud and rain in the centre of the system can still persist for a few more days. The total lifetime of a depression, from frontal wave to occluded system, lasts from a few days to about a week and, in this time, it could easily complete a journey across the North Atlantic. A satellite image of a mature depression is seen in Figure 10.5.

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