As Tropical Cyclone Owen approaches Queensland, here’s everything you need to know
Owen is the first tropical cyclone for the Australian summer and he is starting the season with a bang.
After forming, dying down, crossing from the Coral Sea onto Cape York, moving across to the Gulf of Carpentaria, picking up strength and doing a U-turn, Owen is proving to be a great example of the erratic behaviour of cyclones.
Owen is forecast to cross back into Queensland on Friday morning as a severe tropical cyclone. Its path from there is uncertain but it appears it could head south at least part of the way down the Queensland coast.
So, while you batten down the hatches and prepare for the rain, here is some background on cyclones.
Tropical cyclones, hurricanes and typhoons are all names for the same phenomena. The scales and definitions differ around the world but for general purposes they are all intense, tropical, low-pressure systems with destructive winds.
What makes a cyclone?
Cyclones are part and parcel of an Australian summer. In a normal year, about 11 tropical cyclones develop in Australian waters and four make landfall on the coast.
The spiralling superstorms generally form in the warm waters of the tropics and impact on the northern Australian coast. But as we are forecast to see over the next few days, they can travel down the coast or inland and still have big impacts as ex-tropical cyclones.
Some past examples of cyclones doing damage include Cyclone Debbie in 2017, which caused flooding all the way over the NSW border, Cyclone Oswald in 2013 and Cyclone Wanda in 1974, which caused major flooding in Brisbane.
Tropical cyclones typically cross the coast in the north of Australia but cyclones can still pack a punch as ex-tropical cyclones further south or inland. (ABC Weather: Kate Doyle)
For a cyclone to form there are a few criteria which need to be met:
- Warm waters — more than 26.5 degrees Celsius.
- Humid, rising air to give moisture to the storm.
- Be about 500km or more away from the equator for the Coriolis effect to kick in.
- Low vertical shear.
The Coriolis effect is where the air gets curved as it moves across the globe because the earth rotates faster around the equator than around the poles. In the same way as you get pushed to one side when you try to walk in a straight line on a playground merry-go-round.
Thanks to the earth spinning faster around the equator, cyclones spin clockwise in the southern hemisphere but typhoons and hurricanes spin anticlockwise up north. (ABC Weather: Kate Doyle)
In the southern hemisphere, it means winds are pushed to the left, so as the air is drawn in towards the cyclone, it is pushed to the left and results in a clockwise spiralling motion. In the northern hemisphere, hurricanes and typhoons rotate anticlockwise.
Vertical shear is when a layer of air is going at a different speed or direction to the layer of air above it. Too much shear will rip the storm apart, so it loses momentum.
Even if all of these conditions are met, there is no guarantee a cyclone will form. Cyclones are temperamental, and it can be difficult to tell which little disturbance will go rogue and develop into the next big one.
If a cyclone does form, it will feed off all that heat and moisture and spiral into one of the most powerful natural events on the planet.
The strength of a cyclone is expressed in categories. In Australia, category five cyclones are the strongest, bringing the most destructive winds.
Cyclones can be hundreds of kilometres wide, but the strongest winds are around the eye wall, surrounding the eerily calm ‘eye’ at the centre of the storm.
During a cyclone the wind is fastest right around the eye wall, where the pressure is lowest because the air is being sucked up into the vortex. (ABC Weather: Kate Doyle)
If you are in a cyclone and the wind stops suddenly, don’t come out. You are likely in the eye of the storm, which can take hours to pass over — wait for the all clear before leaving for safety.
The problem with this category system is that it only considers the speed of the winds — not the other impacts of the storm so it can misrepresent the whole impact.
There are other dangers like storm surge, where the cyclone pushes the ocean onto land and can result in water coming up over six metres higher than normal. Storm surge often has the deadliest impact.
Even though the water may not appear deep, you can’t see what is going on under the surface. (ABC Weather: Kate Doyle)
Rain can also bring heavy flooding, before, during and after the winds have settled.
Rainfall can be especially bad if the storm is travelling slowly or if it stops over one area.
So this summer, keep up with the warnings and make sure you have a plan in place for how you are going to respond to an emergency, wherever you are.