This theory, as stated earlier, is also called the frontal theory or the wave theory of the origin of extratropical cyclones.
It was developed by V.Bjerknes and other Norwegian meteorologists working with him at Bergen in the second decade of the twentieth century. It provides an adequate explanation of the origin and development of a middle-latitude cyclone.
This theory recognizes that the polar front, separating polar and tropical air masses, gives rise to cyclonic disturbances that intensify and move along the front and proceed through a somewhat predictable life cycle.
Cyclones, according to Bjerknes, form along a front where polar and tropical air masses with contrasting physical properties (temperature and density) are moving parallel to it in opposite directions.
However, it is to be pointed out that the middle latitudes are an area of convergence and it is here that unlike air masses such as the cold polar air and warm tropical or subtropical air generally meet. It may be noted that the polar front is not a permanent line.
On synoptic charts a polar front may be shown as more than one line. Another characteristic feature of the polar front is the fact that the changing seasons as well as positioning of the jet streams aloft bring about a marked north-south shift in its location.
Any unstable wave originating along these fronts develops, under favourable conditions, into a full-fledged cyclone.
Extratropical cyclones, according to the wave theory, form at a wavelike twist or perturbation on the front. The waves thus formed may be of two types-stable waves and unstable waves.
The stable waves form and dissipate without any visible effect on the weather. In case of unstable waves, they grow in amplitude and go through a cycle as shown in Figure 35.4. According to Byers, unstable waves require a wavelength varying from 500 to 3000 km.
Thus, it is evident that cyclogenesis (cyclone formation) occurs where a frontal surface is distorted into a wave-shaped discontinuity.
There are several factors which may be said to produce an unstable wave on an unperturbed front. Mountains, temperature contrasts or some nearby disturbances may affect the air flow in such a way as to create a wave.
Sometimes, the flow aloft is capable of initiating a cyclonic system. Meandering streams of air in the upper troposphere have been found to intensify the surface cyclonic activity. Surface cyclones occur invariably ahead of an upper-air trough.
According to the polar front theory, as the cold polar air is deflected equator-ward and the warm tropical air pole-ward, a cyclone-forming wave is formed along the front. The wave thus formed is divided into two parts.
The eastern part of the wave where the warm air advancing towards the east ascends over a wedge of cold air mass is called the warm front. The western portion of the discontinuity, where cold polar air is replacing the warm air by under-running the warm and lighter tropical air mass, is the cold front.
Under favourable circumstances, the curbing motion added by the Coriolis force increases the amplitude of the frontal wave. The curvature is also increased. Now, the depression enters into a fully developed stage and has two distinct sectors of warm and cold air.
As the wave develops the ascent of warm air along the warm front results in condensation, cloud formation and precipitation. Since the cold front advances at a faster rate than the warm front, the sector of warm air becomes progressively smaller. The process continues, and the cold front ultimately overtakes the slow moving warm front.
Now, the process of occlusion starts. Whether it is warm-front or cold-front type of occlusion, the warm-air sector is raised aloft, and cold air behind the cold front now meets the cold air in advance of the warm front. It is at this critical point that the cyclone is said to have reached maturity.
With the occlusion of the warm sector, the source of energy for the cyclone is cut off. The centre of occlusion becomes filled with cold air coming in from all directions. The vertical motion stops and there is an all round rise in the surface pressure.
It is noteworthy that when warm and cold fronts are combined into one, a long backward-swinging front is formed. This is called an occluded front.
The onset of the process of occlusion brings the storm close to its death. Thus, continued occlusion hastens the dissipation of the original depression. In the end, the cyclone is a feeble vortex full of fairly uniform air.