Low Pressure

As a tropical cyclone approaches, the barometer falls, at first slowly and then more quickly until it reaches an alarmingly low level. The pressure gradient at sea level near the centre of a mature cyclone is very steep, as shown by closely spaced isobars on a weather chart (Fig. 5.1). It is the steepness of the pressure gradient that accounts for the high wind velocities. Low pressure also has an important influence on storm surge (Section 5.3). The minimum pressure attained is one of two main parameters used to indicate tropical cyclone intensity. The other parameter is wind strength, discussed in Section 5.2.

For South Pacific tropical cyclones, the lowest pressures average around 975 hPa,1 and 50% of storms drop to between 955 and 985 hPa. Exceptional storms of phenomenal intensity may fall well below this level. Tropical Cyclone Zoe is the most intense storm on record, and plummeted to a minimum pressure of 890 hPa on 29 December 2002 when it was located just east of Tikopia in the outer Solomon Islands. Over the last three and a half decades, available data (Fig. 5.2) exhibit periodic oscillation in minimum low pressure values, and a slight downward trend that might be related to global warming and associated sea-surface temperature rise (see Chap. 6).

1 The units of hectopascals (hPa) and millibars (mb) for recording barometric pressure are equivalent.

Close Spaced Isobars
Fig. 5.1. Weather chart showing the closely spaced isobars around Tropical Cyclone Jim, northwest of New Caledonia on 30 January 2006. Courtesy of the Fiji Meteorological Service.

1020

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o 940

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o 940

Linear Isobar Chart

Fig. 5.2. Minimum recorded central pressure of individual tropical cyclones (black markers) from 1970 to 2006, showing the running mean of nine storms (solid line). Note that the markers are spaced equidistantly along the horizontal, so the time axis is non-linear.

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Fig. 5.2. Minimum recorded central pressure of individual tropical cyclones (black markers) from 1970 to 2006, showing the running mean of nine storms (solid line). Note that the markers are spaced equidistantly along the horizontal, so the time axis is non-linear.

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  • amanuel
    What barometer level is a tropical low?
    5 years ago

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