Cyclones

And their effect on the built environment

They are known as Hurricanes in the Caribbean, Typhoons in the China Sea, Cyclones in the Indian Ocean and most commonly called Tropical Cyclones. You can call them what you want but they all have one thing in common: they are extremely destructive when they hit cities and towns.

A cyclone is a huge mass of air moving in a spiral with winds that can reach up to 250 km/h. A cyclone starts off in the sea where the winds are calm and the water temperature is at least 240. Warm air pushes upwards at a fast rate while cold air is sucked in below. The warm air gains moisture and starts producing thunderclouds. Spinning in a clockwise direction (anticlockwise in the Northern Hemisphere), the cyclone gradually picks up speed. Driven by the fast winds, the Cyclone can move at up to 50km/h. While all this is happening a distinct “eye” is forming in the centre of the Cyclone. This “eye” unlike the rest of the Cyclone is completely calm and is not windy at all.

Cyclone Tracy was the most destructive cyclone recorded to hit Australia since white settlement in 1788 (the Bathurst Bay cyclones killed 300 people but didn’t reach land). Cyclone Tracy hit the coastal city of Darwin on Christmas Day 1974. Although people had warning on New Year’s Day about Cyclone Tracy, they were busy preparing for Christmas and thought that the cyclone would pass away just like Cyclone Selma had done three weeks before. Cyclone Tracy came roaring down the Arafura Sea (where most the cyclones develop in that area) and was on track to bypass Darwin but changed direction 900 past Melville Island and straight towards the city. At this time most people were asleep or partying. By 1:00 AM, the city of Darwin felt the full effect of the cyclone. Some people were outside when it hit but most people got to a safe place for shelter. The cyclone had passed by 4:00 AM. Nearly 90% of the houses in the city had been damaged or destroyed (50,000 of the 80,000 were ruined beyond repair) and 66 people were dead. 27 boats went out to sea on the morning of the cyclone to try to ride out the storm, but only 9 returned. 25 out of the 66 people that died, lost their lives at sea. Nearly everyone in the Northern Territorian capital was affected in some way by Cyclone Tracy.

Although you can’t change the path of a cyclone, there are ways of reducing it’s impact. Systems for advanced warning are pretty advanced now and people now have time to prepare for cyclones. There are now satellites and radars so we can detect cyclones and accurately predict their course. There were no accurate warnings 50 years ago and people had to rely on sight and their barometers. Normally most people who die from a cyclone die from being at sea. So if a cyclone is coming, it’s safest to stay indoors in a small room with the windows sealed shut and any debris outside put away in a safe place. The other thing that has changed is that when Darwin was rebuilt after the cyclone, they used new construction rules so that all of the new buildings are much stronger and more likely to survive very strong winds.