The Tropical Cyclone
as explained by New Zealand Weather Ambassador, Bob McDavitt

mcD.jpg (18693 bytes)The idea of naming cyclones sequentially dates back to WW2 when US was bombing Japan. The bomber crews were at it night and day and didn't know one bunch of bad weather from another. During the cyclone season there is a new cyclone every week and often 2 or 3 on the map at once so they were named using the phonetic alphabet (alpha bravo etc). In the late 50s this evolved to a boys names alphabet, and in the late 60s to a boy/girl alphabet. In Guam and the Caribbean the alphabets are reset to A at the start of each season but this is not the practice in the South Pacific (which average about 9 storms per season).

VAUGHAN is just the one of three this year in the Brisbane area (Steve , Tessi, Vaughan) and my memory only reminds me of Leo and Mona in the rest of the South Pacific. There may have been 2 or 3 more perhaps in Nov/Dec 1999...check http://www.solar.ifa.hawaii.edu/tropical/ for more details.

vaughan.gif (440697 bytes)The word HURRICANE is reserved for winds over 63knots (sustained average). The word cyclone just means an area of spinning wind. Its a generic term, even anticyclones are "cyclones" (going the other way).

The term "Tropical Cyclone" is used to refer to Tropical weather systems that are spinning so that they have a ring of gales (at least) near their centre. If it's spinning but not yet a gale it's called a "Tropical Depression". Tropical cyclones that are spinning faster than 63 knots (sustained average) can thus be called Hurricanes (or, in Asia . Typhoons) . Only a few tropical cyclones reach this status. In Australia and around South Pacific the term Hurricane is popular and sometimes even used to describe tropical cyclones that are not strong enough to deserve the name hurricane.

Cyclone are categorised according to the rate they spin - if less than 33knots (sustained average) , its a Tropical Depression, not a cyclone.

Category 1 (Gale)
Strongest gust less than 125 km/h
spinning at 33 to 47 knots = 60 to 90 km/hr
Typical effects (indicative only) - Negligible house damage. Damage to some crops, trees and caravans. Craft may drag moorings.

Category 2 (Storm)
Strongest gust 125 - 170 km/h
spinning at 48 to 63 Knots = 90 to 110 km/hr
Typical effects (indicative only) - Minor house damage. Significant damage to signs, trees and caravans. Heavy damage to some crops. Risk of power failure. Small craft may break moorings.

Category 3 (eg. Winifred)
Strongest gust 170 - 225 km/h (worthy to be called hurricane)
spinning at 64 knots or more = 110 km/hr or more
Typical effects (indicative only) - Some roof and structural damage. Some caravans destroyed. Power failure likely.

Category 4 (eg. Tracy)
Strongest gust 225 - 280 km/h
Typical effects (indicative only) - Significant roofing loss and structural damage. Many caravans destroyed and blown away. Dangerous airborne debris. Widespread power failure.

Category 5 (eg. Orson)
Strongest gust More than 280 km/h
Typical effects (indicative only) - Extremely dangerous with widespread destruction.


Bob McDavitt,
Weather Ambassador for METSERVICE
mcdavitt@met.co.nz