Figure 30 – world: pathways of cyclones (Numbers indicate average frequency of cyclones)

Paths and movement of extra-tropical cyclone – the general direction of movement of temperate cyclones is from west to east with frequent trends towards the southeast to northeast (figure 30). They are subject to the general westerly flow of atmosphere in temperate zone. The heavy concentration of storms tracks in the vicinity of the Aleutian(Islands west to the Alaska Peninsula) and Icelandic lows is the most important feature of the distribution of extra-tropical cyclones. During winter months, the opposing air masses have greater contrasts in their properties. So the winter cyclones are greater in number and are more intense. On an average cyclone may cover a distance of about 1000 km per day. Cyclones invariably move towards higher latitudes.

Secondary cyclones – under the normal conditions, in the later stages of occlusion the cyclone weakens and ultimately dissipates. But sometimes, during the late maturing stage of a cyclone, a new low develops on the equatorward margin of the original cyclone. Thus, a secondary cyclone is formed which passes through different stages of its life cycle and matures very rapidly. It may follow the tract of primary cyclone or may move along new path.

Cyclone families – It is found that an extra-tropical cyclone never appears alone. It is usually followed by three or four cyclones forming a series. The primary or the leading cyclone gets occluded, while the new ones originate on the trailing front and are in an incipient stage. In the rear of the last member of the cyclone family there is an outbreak of polar air which builds up an anti-cyclone. Original cyclone would be in high latitudes and each secondary cyclone would follow progressively a more southerly path. Cyclone families frequent the oceans in a larger number.

Extra-tropical cyclone and Jet stream – there is a close relationship between the flow aloft and the cyclonic storm at the surface. Rossby waves produced at the top of troposphere helps in transporting large bodies of polar air to the lower latitudes and tropical air masses are carried to the higher latitudes. This results in the intensity of surface cyclonic activity. There are instances when extra-tropical cyclones form without the prior existence of a polar front. These depressions are actually initiated by a trough in the upper-air westerlies. Once such storms originate in the lower atmosphere they attract different air masses together which leads to the generation of fronts.