Speed of Advance

Tropical cyclones advance along their track at slow speeds compared to the extremely fast winds (described in Chap. 5) revolving around the storm centre. The speed of cyclone movement is usually under 25 km h-1, equivalent to 600 km or less per day. Yet there is great variability in speed, both between separate cyclones and also during the lifespan of an individual system. As a general rule, a cyclone vortex moves more slowly in the early phase of its life, but then gains speed upon reaching maturity. Tropical cyclones also have a habit of displaying erratic and unpredictable changes in their rate of movement, especially while occupying lower latitudes. They often show a tendency to decelerate, especially if they veer in unusual directions along inflections or looping tracks. Some tropical cyclones stop dead in their tracks and remain almost stationary for a period of several hours, before accelerating once again.

Many cyclones lose their strength and dissipate before leaving tropical latitudes, but those that do survive as they migrate into subtropical and mid-latitudes tend to experience significant acceleration. Vigorous extra-tropical

Cyclone Nancy
Fig. 4.7. Adjacent Tropical Cyclones Olaf and Nancy on 16 February 2005, interfering with each other's movement. Interference between mutually attracted vortices is known as the Fujiwhara effect. Base image courtesy of NASA.

cyclones often travel most rapidly, sometimes reaching double the velocity they had earlier while occupying tropical waters.

Figure 4.8 demonstrates a fairly typical story of several changes in the speed of a tropical cyclone, in this case during the progress of TC Gavin near Tuvalu and through Fiji in early March 1997. As the system developed to the far west of Tuvalu from 2 to 4 March it travelled approximately 450 km in 48 h on an easterly track, from longitude 172°E to 176°E, at an average speed

Fujiwhara Effect

Fig. 4.8. Track of Tropical Cyclone Gavin through Fiji waters in March 1997. Fixes in position of the centre of the eye at 6-hourly intervals in GMT times (dd hh:mm), are shown by the black markers. Changing distances between successive markers illustrate acceleration or deceleration in the speed of the cyclone along its path.

Fig. 4.8. Track of Tropical Cyclone Gavin through Fiji waters in March 1997. Fixes in position of the centre of the eye at 6-hourly intervals in GMT times (dd hh:mm), are shown by the black markers. Changing distances between successive markers illustrate acceleration or deceleration in the speed of the cyclone along its path.

of 9.4 km h-1. Thereafter, TC Gavin accelerated to a steady 20 km h-1 as the storm turned first southeast then south, passing to the west of Wallis and Futuna on 6 March. As TC Gavin approached and subsequently passed the Fiji Islands on a southwesterly course, its speed increased further to 22 km h-1. In contrast, over the next 18 h from 12 midday on 7 March until 6 a.m. on 8 March, the system decelerated rapidly and advanced slowly at an average of

8.5 km h-1, with the track recurving to the southeast. TC Gavin then speeded up once again during the following 24 h, shown by the increasing distance between the 6-hourly fixes, and continued to gain pace while leaving southern Fiji waters on 9 March.

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Responses

  • simone
    What is the velocity of a typical cyclone?
    8 years ago
  • aimone sal
    What is the track speed of tropical storm?
    8 years ago
  • Tommaso Udinese
    Why are cyclones often erratic and unpredictable?
    7 years ago

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