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.