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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) Average 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 - 169
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
170 - 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. One-minute winds have been converted to 10-minute winds using a conversion factor of 0.871. The demarcation points are not precise - this table is intended to provide a rough comparison only.

10-min av. wind speed (knots) 1-min av. wind speed (kn) USA Category Australian Category Wind gusts (km/h) Wind gusts (kn)
57
65
 Category 1 Category 2
148
80
61
70
157
85
65
75
169
91
70
80

 

Category 3
181
98
74
85
 Category 2
193
104
78
90
202
109
83
95
215
116
87
100
 Category 3

 

Category 4
226
122
91
105
235
127
96
110
248
134
100
115
 Category 4
259
140
105
120
272
147
109
125
 Category 5
283
153
113
130
293
158
118
135
306
165
122
140
 Category 5
317
171
126
145
326
176
131
150
339
183

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