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.
KeywordsTropical Cyclone Storm Track Tropical Cyclone Intensity Tropical Depression Cloud Pattern
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