Shape and Size

Atmospheric pressure at sea level at the centre of a tropical cyclone is frequently as low as 965 mb. Away from the centre the pressure increases to about 1,020 mb at a storm's outer edge. This spatial variation in atmospheric pressure at sea level means that one way of examining the shape and size of a tropical cyclone (as with other types of weather systems) is to observe the arrangement of isobars on a synoptic weather chart. The isobar pattern displayed when a storm is slow moving or stationary is often a neat, nearly circular arrangement of concentric rings. This is because there are none of the fronts commonly seen in the structure of mid-latitude depressions (McGregor and Nieuwolt 1998). More rapidly moving tropical cyclones commonly show elliptical or pear-shaped patterns in their isobars (Fig. 3.1). Any elongation in shape is normally oriented in the direction of the storm track (Visher 1925) with the ratio of longest to shortest diameter about 3:2. In elliptical and pear-shaped patterns the isobars are not concentric. There tends to be some bunching of isobars in the leading portion of the storm, relative to storm movement along its track, and some spreading of isobars in the wake. Figure 3.1 illustrates this effect in the isobar patterns of Tropical Cyclones Olaf and Nancy near Samoa and the southern Cook Islands in February 2005.

The diameter of tropical cyclones averages 500-700 km across. Their area, therefore, covers approximately 300,000 km2. Although this is a large area, tropical cyclones are relatively small low-pressure systems compared to mid-latitude depressions, which are often twice as big. Another way of measuring tropical cyclones is in degrees, radially outward from the centre to the outermost closed isobar. The distance in degrees determines the storm's size according to the widely used classification system in Table 3.1. Most South Pacific tropical cyclones fall into the 'medium to average' category, with a radius between 3° and 6°.

Synoptic Tropical Cyclone

Fig. 3.1. Synoptic weather chart of 15 February 2005 showing the isobars around Tropical Cyclones Olaf and Nancy, which are both moving along southeasterly tracks. Note that the shape of the two storms is elongated, with alignment in the direction of movement. This is caused by bunching up of isobars in advance of the storms and spreading out of isobars behind. The shape of TC Nancy is more elliptical and that of TC Olaf more circular. This is owing to the faster speed of TC Nancy, travelling southeast at 15 knots (28 km h-1), compared to TC Olaf's slower progress at 10 knots (19 km h-1). Courtesy of the Fiji Meteorological Service.

Fig. 3.1. Synoptic weather chart of 15 February 2005 showing the isobars around Tropical Cyclones Olaf and Nancy, which are both moving along southeasterly tracks. Note that the shape of the two storms is elongated, with alignment in the direction of movement. This is caused by bunching up of isobars in advance of the storms and spreading out of isobars behind. The shape of TC Nancy is more elliptical and that of TC Olaf more circular. This is owing to the faster speed of TC Nancy, travelling southeast at 15 knots (28 km h-1), compared to TC Olaf's slower progress at 10 knots (19 km h-1). Courtesy of the Fiji Meteorological Service.

Table 3.1. Size classification for tropical cyclones based on radius in degrees, measured from the storm centre to the outermost closed isobar.

Radius in degrees Tropical cyclone size

<2 Very small (midget)

2-3 Small

3-6 Medium to average 6-8 Large

>8 Very large

+2 -1

Responses

  • Elias
    What is the shape and size of the tropical cyclones?
    2 months ago

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