Cyclones

The cyclone season is from November to April. Active monsoon periods are likely times for development of cyclones.

Cyclones form over warm ocean water, in an area of low pressure. These conditions are generally found within 10-20° latitude on either side of the equator, where water temperatures of at least 26°C occur, making the oceans around the northern half of Australia perfect.

Resembling a giant whirlwind, the winds of a cyclone in the southern hemisphere spiral clockwise towards its centre, where they rise up to the top of the cloud mass and spill over into the upper atmosphere. The cyclone usually grows in size and strength as it travels at moderately low speeds of 20-25km/hr.

The sun's heat evaporates the ocean's warm water, with the moist air cooling and condensing as it rises, ultimately producing torrential rain. These rains can cause enormous damage through flooding and landslides.

How much damage will a cyclone cause?

The extent of the damage depends on the force of the winds, the cyclone's path, what it picks up along the way, and how well prepared you are. Remember, whatever it picks up, it throws down later with enormous force (and that could be your roof).

Australia's most destructive cyclone, Tracy, which destroyed much of Darwin in December 1974, recorded winds in excess of 217km/hr, with 195mm of rainfall recorded in less than nine hours.

To find out what to expect from different cyclone categories, click here.

Beware the eye of the cyclone

If you've ever seen images of cyclones, they all look different. But inside, they all look much the same. The centre of the cyclone, or the eye, is the point where the pressure is at its lowest.

The eye is commonly 40km in diameter, but can reach up to 100km across. Unlike the rest of the cyclone, in the eye, light winds and clear skies prevail. But beware, this can be deceivingly dangerous!

If the eye passes directly over you, you'll experience a sudden lull in wind, which may last for a few minutes or up to two hours. Hence, people often think the cyclone has passed - but that was only half of it. The other side of the cyclone then strikes, with winds blowing again, but this time from the opposite direction.

The area immediately surrounding the eye is called the wall. This dense ring of cloud reaches upwards of 15km high. It marks the region of the cyclone's strongest winds and heaviest rainfall, and causes most of the damage.

Warning Systems

It is absolutely vital that you understand Cyclone Advices and Warnings. It might save your life. Advices are issued from the Tropical Cyclone Warning Centre (TCWC) of the Bureau of Meteorology in Darwin. Advices consist of Cyclone Watches and Cyclone Warnings.

Cyclone Watch - 48 hours before

If information indicates that the cyclone will affect coastal or island communities within 48 hours, but not within 24 hours, a Cyclone Watch is issued outlining the cyclone's location, movements, strength and the areas that could be affected. A Cyclone Watch is updated every 6 hours.

Cyclone Warning - 24 hours before

A Cyclone Warning is issued as soon as gales or stronger winds are expected to affect coastal or island communities within 24 hours. A Cyclone Warning outlines the cyclone's location, movements, strength and the areas being threatened. Forecasts of heavy rainfall, flooding and abnormally high tides are included where necessary.

Communities under threat are advised of immediate precautions necessary to safeguard their lives and property. A Cyclone Warning is updated every three hours, with hourly Warnings issued if it moves close to the coast, posing a major threat. Make sure you listen to your radio and follow all official warnings and advice. Please help friends and neighbours, particularly the elderly, by passing on the warnings and their meaning.

To check on any current cyclone activity, click on one of the links below.

Cyclone Categories

Prediction - Preparation - Protection
Cyclones are extremely unpredictable, but in most cases, the Bureau of Meteorology can predict where and when a cyclone will do its worst. Throughout Australia, the Bureau is responsible for detecting, tracking and issuing cyclone warnings. You are responsible for ensuring the safety of your family, yourself and your property.

Prediction
A cyclone can last for days or even weeks. It may stay still, move forward, move in circles, or even back-track. Whatever it does, the Bureau is tracking its every move. Scientists and technical staff use radar, satellite, weather stations and sophisticated computer models to detect and assess cyclone threat. They analyse the data and communicate information and warnings to police, State Emergency Service groups and media in threatened areas.

Preparation
People living in tropical coastal regions should always be conscious of a cyclone threat and be prepared. Simple measures taken before this time will give you peace of mind now, rather than panic later.

The following table shows you what to expect from different cyclone categories.

Tropical Cyclone Severity Categories				Typical Effects (indicative only)
Category	Average Winds Speed (kmh)	Strongest Gust (kmh)	Central Pressure (hPa)	Typical Effects General	General Water Facilities	General Sewerage Facilities	Power
1	63-90	Below 125	Over 985	Negligible house damage. Damage to some crops, trees and caravans. Craft may damage moorings.	Negligible damage to facilities.	Reliable damage to facilities. Some minor sewage reticulation overflows.	Minor outages (hours).
2	90-120	125-170	985-970	Minor house damage. Significant damage to signs, trees, caravans. Heavy damage to crops. Small craft may break moorings.	Minor damage to facilities. A number of trees falling over damaging services and uprooting pipes.	Minor damage to facilities. A number of sewage overflows from reticulation.	Risk of power failure. Moderate outages (hours -days).
3	120-160	170-225	970-945	Some roof and structural damage. Some caravans destroyed.	Some damage to facilities including 300 and 375 mains	Some damage to switchboards on minor pumpstation. Sewage overflows throughout reticulation and minor pumpstations.	Power failure likely Major outages (days - weeks).
4	160-200	225-280	945-920	Dangerous airborne debris. Significant roofing loss and structural damage. Many caravans destroyed.	Damage to facilities and roof on ground tanks. Most above ground fire hydrants and banjo connections broken.	Damage to switchboards on minor pumpstation. Widespread sewage overflows.	Widespread power failure likely Extensive outages (weeks).
5	Over 200	Over 280	Below 920	Extremely dangerous with widespread destruction.	Loss of roof on ground tanks. Widespread damage to reticulation from falling trees, flying debris.	Destruction of switchboards on all minor pumpstations. Extensive sewage overflows and damage to reticulation from falling trees. High potential for storm surge damage to treatment facilities. Ludmilla lime storage destroyed and other structural damage.	Complete outages (months).

Storm Surges

Cyclones can create storm surges, which usually means increased flooding for coastal areas. Unlike a tidal wave, which comes crashing into the shore as one wall of water, a storm surge reaches the coastline like a rapidly rising tide, reaching two to five metres above the normal sea level, and affecting a coastal zone up to 100km across.

All cyclones produce storm surges, but they're not always dangerous. It depends on the cyclone's strength, speed, shape of the sea floor, features of the land, angle it crosses the coast, and most importantly, when it makes landfall. If a storm surge occurs at high tide, flooding will be at its worst.

Click here for more information on Storm Surges