Tropical Cyclone Intensity and Impacts

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Tropical Cyclone Intensity

Tropical cyclone intensity is defined by the maximum mean wind speed over open flat land or water. This is sometimes referred to as the maximum sustained wind and will be experienced around the eye-wall of the cyclone.

Mean Winds and Gusts

Mean Wind: In most of the world the mean wind speed is defined as the wind speed averaged over a period of 10 minutes. It should be measured at 10 m above the surface. The major exception is the USA where they use a 1-minute average.

Wind Gust: In most of the world the wind gust speed is defined as the wind speed averaged over 2 or 3 seconds (in Australia we use 3 seconds).

Typically gusts over open land will be about 40% greater than the mean wind and gusts over the ocean will be 25 - 30% greater than the mean wind. It is often the stronger gusts that cause the most significant damage to buildings

While a cyclone advice may refer to a certain maximum sustained wind or gust, there will be localised points where the winds will exceed this value, particularly in gullies, about ridges and between buildings where winds can be funnelled by the landscape.

Extent of Significant Winds

The extent of damaging winds will vary between cyclones. More importantly, the most severe winds will be confined to a small area around the outside of the eye.Often people will experience the winds in the outer part of a Category 4 or 5 cyclone. They will believe that they have experienced a major cyclone, yet the winds may have only been Cat 1 or 2 strength.

It is important to recognise the structure of a cyclone when assessing past experience. This will make a future direct hit less of a surprise.

Diagram of cyclone structure

Tropical Cyclone Category System

CATEGORY 1 (tropical cyclone)
Negligible house damage. Damage to some crops, trees and caravans. Craft may drag moorings.
A Category 1 cyclone's strongest winds are GALES with typical gusts over open flat land of 90 - 125 km/h.
These winds correspond to Beaufort 8 and 9 (Gales and strong gales).
CATEGORY 2 (tropical cyclone)
Minor house damage. Significant damage to signs, trees and caravans. Heavy damage to some crops. Risk of power failure. Small craft may break moorings.
A Category 2 cyclone's strongest winds are DESTRUCTIVE winds with typical gusts over open flat land of 125 - 164 km/h. These winds correspond to Beaufort 10 and 11 (Storm and violent storm).
CATEGORY 3 (severe tropical cyclone)
Some roof and structural damage. Some caravans destroyed. Power failures likely.
A Category 3 cyclone's strongest winds are VERY DESTRUCTIVE winds with typical gusts over open flat land of 165 - 224 km/h.
These winds correspond to the highest category on the Beaufort scale, Beaufort 12 (Hurricane).
CATEGORY 4 (severe tropical cyclone)
Significant roofing loss and structural damage. Many caravans destroyed and blown away. Dangerous airborne debris. Widespread power failures.
A Category 4 cyclone's strongest winds are VERY DESTRUCTIVE winds with typical gusts over open flat land of 225 - 279 km/h.
These winds correspond to the highest category on the Beaufort scale, Beaufort 12 (Hurricane).
CATEGORY 5 (severe tropical cyclone)
Extremely dangerous with widespread destruction.
A Category 5 cyclone's strongest winds are VERY DESTRUCTIVE winds with typical gusts over open flat land of more than 280 km/h.
These winds correspond to the highest category on the Beaufort scale, Beaufort 12 (Hurricane).

The Beaufort Scale

Beaufort scale Cyclone category Average wind speed (knots) Average wind speed (km/h) Estimating speed over land Estimating speed over water
0 Calm   Less than 1 less than 1 Calm, smoke rises vertically. Sea like mirror
1 Light Air   1 - 3 1 - 5 Direction of wind shown by smoke drift, but not by wind vanes. Ripples with the appearance of scales are formed, but without foam crests
2 Light breeze   4 - 6 6 - 11 Wind felt on face; leaves rustle; ordinary wind vane moved by wind. Small wavelets, still short, but more pronounced; crests have a glassy appearance and do not break
3 Gentle breeze   7 - 10 12 - 19 Leaves and small twigs in constant motion; wind extends light flag. Large wavelets; crests begin to break; foam of glassy appearance; perhaps scattered white horses
4 Moderate breeze   11 - 16 20 - 28 Raises dust and loose paper; small branches moved. Small waves, becoming longer; fairly frequent white horses
5 Fresh breeze   17 - 21 29 - 38 Small trees in leaf begin to sway; crested wavelets form on inland waters. Moderate waves, taking a more pronounced long form; many white horses are formed (chance of some spray)
6 Strong breeze   22 - 27 39 - 49 Large branches in motion; whistling heard in telegraph wires; umbrellas used with difficulty. Large waves begin to form; the white foam crests are more extensive everywhere (probably some spray)
7 Near gale   28 - 33 50 - 61 Whole trees in motion; inconvenience felt when walking against the wind. Sea heaps up and white foam from breaking waves begins to be blown in streaks along the direction of the wind
8 Gale 1 34 - 40 62 - 74 Breaks twigs off trees; generally impedes progress. Moderately high waves of greater length; edges of crests begin to break into the spindrift; the foam is blown in well-marked streaks along the direction of the wind
9 Strong gale 1 41 - 47 75 - 88 Slight structural damage occurs (chimney pots and slates removed). High waves; dense streaks of foam along the direction of the wind; crests of waves begin to topple, tumble and roll over; spray may affect visibility
10 Storm 2 48 - 55 89 - 102 Seldom experienced inland; trees uprooted; considerable structural damage occurs. Very high waves with long overhanging crests; the resulting foam, in great patches, is blown in dense white streaks along the direction of the wind; on the whole, the surface of the sea takes a white appearance; the tumbling of the sea becomes heavy and shock-like; visibility affected
11 Violent storm 2 56 - 63 103 - 117 Very rarely experienced; accompanied by widespread damage. Exceptionally high waves (small and medium sized ships might be for a time lost to view behind the waves); the sea is completely covered with long white patches of foam lying along the direction of the wind; everywhere the edges of the wave crests are blown into froth; visibility affected
12 Hurricane 3,4,5 64 and over 118 and over Severe and extensive damage. The air is filled with foam and spray; sea completely white with driving spray; visibility very seriously affected

Global Tropical Cyclone Terminology

Tropical cyclones can be defined in different ways elsewhere in the world. Often news reports from the United States or Asia will refer to hurricanes or typhoons. These are all tropical cyclones, but with different names. While the category definitions are not identical, the following provides an approximate guide for comparison

Australian name Australian category US* US Saffir-Simpson category scale* NW Pacific Arabian Sea /Bay of Bengal SW Indian Ocean South Pacific (East of 160E)
Tropical low - Tropical depression - Tropical depression Depression or severe depression Tropical depression Tropical depression
Tropical cyclone 1 Tropical storm - Tropical storm Cyclonic storm Moderate tropical storm Tropical cyclone (Gale)
Tropical cyclone 2 Tropical storm - Severe tropical storm Severe cyclonic storm Severe tropical storm Tropical cyclone (Storm)
Severe tropical Cyclone 3 Hurricane 1 Typhoon Very severe cyclonic storm Tropical cyclone Tropical cyclone (Hurricane)
Severe tropical cyclone 4 Hurricane 2 - 3 Typhoon Very severe cyclonic storm Intense tropical cyclone Tropical cyclone (Hurricane)
Severe tropical cyclone 5 Hurricane 4 - 5 Typhoon Super cyclonic storm Very intense tropical cyclone Tropical cyclone (Hurricane)

* Note that the USA uses 1-minute wind averages, whichare generally greater than 10-minute wind averages used elsewherein the world – hence their intensity definitions (windstrengths) will differ by about 10%.