What is a cyclone? 

Violent wind storms are called cyclones in Australia, and are known as typhoons or hurricanes in other countries. 

A cyclone begins over warm tropical seas when a large mass of air rotates around an area of low atmospheric pressure. The warm, moist air becomes a strong, circling, wind storm.

Cyclones spiral clockwise in the southern hemisphere and anti-clockwise in the northern hemisphere.

The eye of the storm

In the centre of a cyclone, there is a calm area called the 'eye'. In the eye, the sky is clear and there is only a light wind blowing.

Cyclones can change direction suddenly, which makes it very difficult for meteorologists to forecast what will happen.

 Cyclones move at up to 360 kilometres per hour, bringing heavy rain and very high winds and causing high waves. The wind and rain cause lots of damage when the cyclone crosses over coastal lands.

Cyclone Tracy

The worst cyclone in Australia was Cyclone Tracy, which devastated the city of Darwin, Northern Territory on Christmas Eve 1974.

On December 20, 1974 the Bureau of Meteorology in Northern Australia noticed a cyclone forming off the coast. They named it Tracy. 

The people at the weather bureau closely watched Tracy for the next few days but did not feel that the cyclone posed a major threat to Darwin. They thought that it would pass well to the north of Darwin.

However early on Christmas Eve, 24 December 1974, Tracy came closer, turned and began to speed towards the city. From midnight until 7am on Christmas Day the cyclone passed directly over Darwin. The wind speed was 217 kilometres per hour and Tracy brought huge rainfall. Houses and other buildings were blown apart, 49 people died in the city and another 16 were drowned at sea. Seven out of every ten of Darwin's homes were destroyed or severely damaged.

Communications, power, water and sewerage services were all broken. Emergency committees worked to provide accommodation, food and clothing to all those whose houses had been destroyed or damaged. Australia's defence forces took a major role in cleaning up the city and suburbs.

There was a threat of disease too because sewerage systems had been destroyed. In all, 25 000 people were evacuated to southern cities.

Between 1975 and 1976 over 150 million dollars was spent to build or repair more than 2500 homes as well as other many other buildings.

Watch a video about hurricanes (cyclones) in action :

What is a tornado?

A tornado is a violent, twisting windstorm. A tornado forms when air in a thunder cloud is set spinning by wind, and more and more air is sucked up into a spinning column like a giant vacuum cleaner.

Tornadoes cause great damage. They suck up dust, uproot trees, overturn cars and buildings, and lift roofs and even people.

Tornadoes in deserts are sometimes called 'dust devils', and in Australia, an Aboriginal name for a tornado is 'willy-willy'. 

How Does a Tornado Form? 

The Beaufort Wind Scale

This is one way that the strength of the wind is measured. The scale is named after Sir Francis Beaufort, who developed it in 1805 for use by sailors. You can use the Beaufort scale to measure the force of the wind by looking at the effects of the wind on things around you.

Scale          description:           wind speed:           You may see: 
Force 0          calm                     0 km per hour      Smoke rises vertically

Force 1          light air                1-5 kph               Smoke blown by wind

Force 2         light breeze          6-11 kph               Leaves rustle, feel the wind on face

Force 3          gentle breeze     12-19 kph             Moving  leaves & twigs;  flags flutter

Force 4         moderate breeze 20-30 kph             Wind raises dust and fallen leaves. Paper                                                                                      blows around. Small tree branches sway
Force 5           fresh breeze       31-39 kph             Small trees begin to sway

Force 6        strong breeze       40-50 kph             Large branches sway. Umbrellas hard to                                                                                        hold, blown inside out

Force 7        near gale              51-61 kph             Whole trees sway. Feel the wind push you

Force 8        gale                        62-74 kph            Difficult to walk in the wind. Twigs broken                                                                                       off trees
Force 9         strong gale           75-87 kph       Tiles blown off roofs, branches blown down

Force 10       storm                    88-102 kph        Houses damaged. Whole trees broken or                                                                                     blown over
Force 11      violent storm        102-116 kph       Serious damage

Force 12      cyclone                   117-132 kph    Widespread damage