The name cyclone means a turning wind with one eye, it is a term derived from the name Cyclops, a one-eyed creature from ancient Greek mythology In meteorological terms a cyclone refers to an area of closed circular fluid motion almost always centred on an area of low atmospheric pressure. Some cyclones have a cold core (eg polar cyclones) but the cyclones we are familiar with in Queensland December to April are warm core cyclones.
In North America tropical cyclones are known as hurricanes (from the name of a wind deity Huracan ) and in Asia as typhoons. (Big wind in Chinese)
Tropical cyclones form over very warm seas that have a temperature above 26.5 degrees C to a depth of at least 50 metres. There must also be high humidity, atmospheric instability, and a pre-existing point of disturbance and strong wind gradient extending to the upper atmosphere.
Another vital factor in the dynamics of cyclones is the Coriolis effect, which describes motion in a rotating term of reference. The Earth rotates on a north south axis and air flowing to a point of low pressure tends to spiral anti-clockwise in the Northern Hemisphere and clock-wise in the Southern Hemisphere. The shape of the earth effects the strength of the Coriolis forces; it is weakest at the equator and strongest at the poles.
Within approximately 8 degrees from the equator the Coriolis force is too weak to establish the circular equilibrium of a cyclone.
The energy driving a tropical cyclone is derived from convection and condensation at the centre of the storm. The moist hot air above the warm sea rises into the atmosphere; the water vapour condenses and releases latent heat into the upper atmosphere. Colder air replaces warm air above the sea and this convection cycle intensifies, causing thunderstorms, high winds, turbulence, torrential rainfall, rough seas and storm surges.
The low pressure at the centre sucks up the surface of the sea and this, plus the effect of wind and waves can lift the surface of the sea over 5 metres. The highest storm surge recorded was 14.6 m produced by Cyclone Mahina in 1899, after the storm, fish and dolphins were found on top of 15m cliffs. The highest wave recorded was by an American warship during a typhoon in 1933, it measured 34 metres.
In the centre of the cyclone is its eye, which may be from 10 to 100km across. The widest recorded cyclone was Tip at 1100 km and the smallest was Cyclone Tracy at only 50 km wide.Once a cyclone passes over colder seas or over land it tends to dissipate.
The Saffir-Simpson scale classifies cyclones into categories 1 to 5 based on wind speed and storm surge, Category 1 wind-speed above 119 km/h and storm surge above 1.2 m. Category 5 wind-speed above 250 km/h and storm surge above 5.5 m
Cyclones cause devastation but they are also a key component in our weather system.