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Comparison between the Australian Tropical Cyclone Intensity Scale and the Saffin/Simpson Hurricane Intensity Scale
by Gary Padgett

Both the U. S. National Weather Service and Australia's Bureau of Meteorology (BoM) employ a 5-point scale by which to alert the public of the general effects which can be expected from tropical cyclones of varying intensities. In both systems tropical cyclones are classified from Category 1 (the weakest) to Category 5 (the strongest), and each scale seems to have been adopted enthusiastically by the general public in both Australia and in the United States.

However, the scales are very different in their conception and in the boundaries between their respective categories. The Saffir/Simpson scale, used in the United States, is used for classifying tropical cyclones which have reached hurricane intensity while the Australian scale is utilized for all tropical cyclones of gale intensity or higher. Nowadays, anyone anywhere in the world with internet access can follow tropical cyclones in great detail, and there are many persons who delight in tracking and discussing storms in all oceanic basins. Considerable confusion has sometimes arisen when, for example, someone in the U. S. is following an Australian cyclone and assuming that the category given in the BoM warnings (and often aired in the international news media) is equivalent to its Saffir/Simpson category.

The following chart summarizes the salient characteristics of the two scales:

Characteristic Saffir/Simpson Australian
Beginning Point Hurricane force Gale force
Wind Parameter Sustained wind (1-min avg) Peak gusts
Wind Velocity Units Statute miles-per-hour Kilometres-per-hour

My purpose here is to provide information whereby persons interested in comparing the two scales can easily do so. Table 1 defines the Saffir/Simpson Scale in terms of the 1-min avg maximum sustained wind (MSW) in statute miles-per-hour (mph) while Table 2 defines the Australian Cyclone Severity Scale in terms of peak wind gusts in kilometres-per-hour (km/hr). I have not included the narrative describing the degree and type of damage which can be expected with each category in both scales. Also, in both tables I have included the qualifying wind velocity in nautical miles-per-hour (kts) along with the nominal central pressure range in millibars (mb)--numerically equal to hectopascals (hPa).

Table 1 - Saffir/Simpson Scale

Category S/S 1-min avg MSW (mph) 1-min avg MSW (kts) Central Pressure (mb)
1 74 - 95 64 - 82 >= 980
2 96 - 110 83 - 95 965 - 979
3 111 - 130 96 - 113 945 - 964
4 131 - 155 114 - 135 920 - 944
5 >= 156 >= 136 <= 919

Table 2 - Australian Cyclone Severity Scale

Category Aust Peak gusts (km/hr) Peak gusts (kts) Central Pressure (hPa)
1 90 - 124 49 - 67 986 - 995
2 125 - 169 68 - 91 971 - 985
3 170 - 224 92 - 121 956 - 970
4 225 - 279 122 - 150 930 - 955
5 >= 280 >= 151 <= 929

The following two tables define each scale in terms of the other scale's parameters, i.e., the Saffir/Simpson scale is defined in terms of a 10-min avg MSW in kts, and peak gusts in kts and in km/hr (Table 3) while the Australian scale is defined in terms of a 10-min avg MSW in kts and a 1-min avg MSW in mph (Table 4).

Table 3 - Saffir/Simpson Scale in Terms of Australian Scale Parameters

Category S/S 10-min avg MSW (kts) Peak gusts (kts) Peak Gusts (km/hr) Equivalent Aust Category
1 56 - 72 79 - 101 146 - 190 mid-2 to mid-3
2 73 - 84 102 - 118 191 - 220 mid to high-3
3 85 - 99 119 - 140 221 - 260 high-3 to mid-4
4 100 - 118 141 - 166 261 - 310 mid-4 to mid-5
5 >= 119 >= 167 >= 311 mid to high-5

Table 4 - Australian Scale in Terms of Saffir/Simpson Scale Parameters

Category Aust 10-min avg MSW (kts) 1-min avg MSW (kts) 1-min avg MSW (mph) Equivalent S/S Category
1 34 - 47 38 - 54 44 - 62 none
2 48 - 64 55 - 73 63 - 84 none to mid-1
3 65 - 85 74 - 96 85 - 111 mid-1 to high-2
4 86 - 106 97 - 120 112 - 139 low-3 to low-4
5 >= 107 >= 120 >= 140 mid-4 and up

Finally, as an additional aid to comparison, I have included a table (Table 5) comparing each cyclone scale with the Dvorak scale, used extensively in estimating tropical cyclone intensity from visible and infrared satellite imagery. I have started the table with T2.0 since neither TPC/NHC nor the BoM TCWCs routinely issue advisories/ advices/warnings for systems less than that Dvorak rating. (See the key following the table for an explanation of the abbreviations used.)

Table 5 - Comparison of Dvorak Scale with Cyclone Scales

Dvorak T-Number MSW (1-min kts) MSW (10-min kts) TPC/NHC Category Australian Category
T2.0 30 26 TD TL
T2.5 35 31 TS TL
T3.0 45 40 TS TC-1
T3.5 55 48 TS TC-2
T4.0 65 57 H-1 TC-2
T4.5 77 68 H-1 TC-3
T5.0 90 79 H-2 TC-3
T5.5 102 90 H-3 TC-4
T6.0 115 101 H-4 TC-4
T6.5 127 112 H-4 TC-5
T7.0 140 123 H-5 TC-5
T7.5 155 136 H-5 TC-5
T8.0 170 150 H-5 TC-5

  Key to Abbreviations:
  ---------------------
  TD      tropical depression
  TL      tropical LOW
  H-n     hurricane of Saffir/Simpson category "n"
  TC-n    tropical cyclone of category "n" on Australian scale

Most of the information presented above was obtained from the websites of Australia's Bureau of Meteorology and the Tropical Prediction Center/National Hurricane Center.

Perth Tropical Cyclone Warning Centre also has a Intensity Scale Comparison.


Document: intensity_scale.htm
Updated: 3rd May 2006

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