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Definitions

2. What is the tropical cyclones intensity scale? How is this different from the USA intensity scale?

The severity of a tropical cyclone is described in terms of categories ranging from 1 (weakest) to 5 (strongest) related to the zone of maximum wind gusts as shown in this table.
Note: corresponding sustained winds and central pressure are also provided as a guide.

Category Strongest Gust (km/h) Sustained
(10 minute) Maximum Wind (km/h)
Approximate Central Pressure (hPa) Typical Effects
1
< 125
63 - 88
> 985
Negligible house damage. Damage to some crops, trees and caravans. Craft may drag moorings
2
125 - 164
89 - 117
985 - 970
Minor house damage. Significant damage to signs, trees and caravans. Heavy damage to some crops. Risk of power failure. Small craft may break moorings.
3
165 - 224
118 - 159
970 - 955
Some roof and structural damage. Some caravans destroyed. Power failures likely. (e.g. Winifred)
4
225 - 279
160 - 199
955 - 930
Significant roofing loss and structural damage. Many caravans destroyed and blown away. Dangerous airborne debris. Widespread power failures. (e.g. Tracy, Olivia)
5
> 279
> 200
< 930
Extremely dangerous with widespread destruction. (e.g. Vance)

The USA utilizes the Saffir-Simpson hurricane intensity scale for the Atlantic and Northeast Pacific basins. Go to the US Atlantic Oceanographic and Meteorological Laboratory's Hurricane FAQ page for more information about this scale.

The table below provides a comparison of the USA hurricane scale with the Bureau of Meteorology's 5-point tropical cyclone categories. A complicating factor is that the wind averaging period used in the USA is one minute whereas it is ten minutes in the Australian region. One-minute winds have been converted to ten minute winds using a conversion factor of 0.88. The associated wind gusts are also listed, the conversion factor from the ten minute winds being 1.41. The demarcation points are not precise - this table is intended to provide a rough comparison only.

1-min av. wind speed (knots)
USA Category
Australian Category
10-min av. wind speed (knots)

Wind gusts
(knots)

Wind gusts (km/h)
65
Category 1
Category 2
57
81
149
70
62
87
161
75

 

Category 3

66
93
172
80
70
99
184
85
Category 2
75
105
195
90
79
112
207
95
84
118
218
100
Category 3

 

Category 4

88
124
230
105
92
130
240
110
97
136
253
115
Category 4
101
143
264
120
106
149
275
125
Category 5
110
155
287
130
114
161
298
135
119
168
310
140
Category 5
123
174
321
145
128
180
333
150
132
186
344

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