After various conversations with numerous people over the years, it’s become quite evident to me that the vast majority of locals (both new to the area and long time locals) are greatly unaware and uneducated about cyclones. I’m not picking on anyone, but a little bit of information can be beneficial to us all. So with the threat of a potentially very active season looming, I thought I might take the opportunity to try and explain some of common issues I often encounter. Of course I’m not a cyclone “expert” and certainly don’t claim to be, so if others have something worthwhile to add, please don’t hesitate.
Righteo, starting off with the basics…. in case you didn’t already know, cyclones are large rotating systems originating over tropical waters and are capable of producing very heavy rainfall, storm surges and wind speeds in excess of 300km/h (in the most severe cases).
Of course here in Australia, cyclones are rated by categories numbered 1-5 based on how strong the highest wind gusts are estimated to be within the cyclone’s zone of greatest winds (just outside the cyclone’s eye).
Category 1 = less than 125km/h
Category 2 = 125 – 164km/h
Category 3 = 165 – 224km/h
Category 4 = 225-279km/h
Category 5 = more than 280km/h
(Note, Tropical Cyclones of category 3 or higher intensity are called “Severe Tropical Cyclones”.)
Anyway I won’t get too stuck into this detail as this type of information is available on the BoM’s website. Click HERE
OK now then, getting to some of the issues and misunderstandings…… A few of months ago I was talking to a bloke I know (55 years old and lived in Mackay all his life). He was quite surprised by the winds he experienced during Cyclone Ului (March 2010). But it wasn’t the strength of the winds that had him confused, rather the direction they were blowing from. Living 120km south of where Ului made landfall, naturally he expected the winds would blow from the north. Makes sense doesn’t it? Cyclone to the north = winds will blow from the north. WRONG. Because cyclones are large rotating systems, depending on where you are positioned in regards to the centre will determine which direction the winds will be blowing from. In the Southern Hemisphere winds rotate clockwise around Cyclones.
So if a cyclone is in your vicinity, as a rough guide:
If the Centre of the Cyclone is located to your NORTH, winds will blow from the EAST
If the Centre of the Cyclone is located to your NORTH-EAST, winds will blow from the SOUTH-EAST
If the Centre of the Cyclone is located to your EAST, winds will blow from the SOUTH
If the Centre of the Cyclone is located to your SOUTH-EAST, winds will blow from the SOUTH-WEST
If the Centre of the Cyclone is located to your SOUTH, winds will blow from the WEST
If the Centre of the Cyclone is located to your SOUTH-WEST, winds will blow from the NORTH-WEST
If the Centre of the Cyclone is located to your WEST, winds will blow from the NORTH
If the Centre of the Cyclone is located to your NORTH-WEST, winds will blow from the NORTH EAST
Bear in mind, as the cyclone moves the wind direction will change accordingly. Therefore if a cyclone passes in your vicinity, the wind may change direction several times in just a matter of hours.
During a cyclone if you notice winds increase in violence for several hours then suddenly ease to almost calm conditions, contrary to many peoples belief, IT DOES NOT MEAN THE CYCLONE HAS ENDED – rather you are currently in the calm eye (centre) of the cyclone. During this time it is NOT advised that you venture outside as winds will again return to their previous strength from a different direction. There is no set rule for how long calm conditions will remain for. Several factors determine how long the calm will last. Some of these include…….
• The size of the eye:
Most cyclones have an eye diameter of approximately 20 - 40km. However the eye of some cyclones can be as small as 5km, and the diameter of the largest eye on record is 320km.
• Your location within the eye:
The calm conditions would last longer if the very centre of the eye passed over you, rather than if you were located only just within the edge of the eye.
• The speed the cyclone is moving:
Cyclones move at different speeds. Generally I would class a slow moving cyclone one that travels less than 10km/h, and a fast moving cyclone one that travels more than 25km/h. If I recall correctly, when cyclone Larry made landfall he was travelling at 30km/h. Obviously the faster the cyclone is moving, the less time it will take for the eye to pass over - therefore reducing the duration of calm conditions.
I might also add that a cyclone moving at a slow speed will cause more damage than an identical cyclone moving at a faster speed as the destructive winds within the core will take longer to pass by. i.e. a house will suffer more damage during 200km/h winds lasting 4 hours rather than only 1 hour.
Another belief we can throw out the window is………
I often hear people say “if a cyclone moves faster, it will lose intensity”. THIS IS NOT ALWAYS TRUE. Yes in many cases this will occur, particularly those off the Qld coast that are caught by strong upper level North-Westerly winds and pushed away to the land of the sheep shaggers. But under favourable conditions cyclones are capable of accelerating while still intensifying. A recent example of this was just earlier this year when Cyclone Ului re-intensified to a category 3 cyclone just prior to landfall, while travelling at a speed of 27km/h and continuing to accelerate. So don’t always assume that if a cyclone is increasing in speed that it will lose its intensity.
Another lesson that is important to remember is:
If you have experienced a cyclone in the past, although the cyclone may have been ranked a category 4 system as it made landfall, it doesn’t necessarily mean the winds you experienced at your location were of category 4 strength.
Cyclone Ului made landfall as a category 3 system at Airlie Beach with winds near Midge point probably in the vicinity of 180km/h. Yet about 70km to the south, the highest wind gust recorded in Mackay was only 113km/h – which is only the equivalent of a category 1 cyclone. Despite this, I can guarantee you there are hundreds of people in Mackay thinking they experienced winds equivalent of a category 3 cyclone. The bad news is, if another cyclone was to hit Mackay next week as a category 2 system (less intense) those people would assume “Well if we had no troubles with a category 3 cyclone, then if we were hit by a category 2 cyclone there would be even less damage”. THIS IS NOT NECESSARILLY TRUE. This is because Mackay was located outside the zone of maximum winds; therefore Mackay didn’t feel the full force of the cyclone. So although a Category 2 cyclone may seem tamer than category 3, depending on where is makes landfall, and how far the radius of maximum winds extend from the centre will determine what the wind speeds will be like at your location.
This brings me to another important issue. No two cyclones are the same – they all have their own individual “personalities” (i.e., some are big, some are small, some are fast, some are slow, some are predictable, others are unpredictable, some live longer than others, and the list goes on..…..) One of the biggest issues here is that some cyclones are larger than others. When I say “larger” what I mean is that the area of destructive and gale force winds extend to a much greater diameter.
Generally, most Qld cyclones have gales (winds above 63km/h) extending about 200km from the centre of the cyclone. In 1974, gales from Cyclone Tracy (arguably Australia’s most significant cyclone from the last 100 years) only extended 50km from the eye. On the other end of the scale, at one stage Cyclone Justin (1997) had gale force winds extended more than 700km from the centre of the cyclone.
So for another example….
When Cyclone Larry made landfall at Innisfail as a category 4 cyclone in 2006, Townsville (about 200km away) recorded a maximum wind gust of 71km/h. Yet in 1976, Category 3 (less intense) cyclone John made landfall just north of St. Lawrence, but managed to produce a wind gust of 156km/h at Gladstone (also 200km south of where the cyclone passed by). So if you live in Townsville, and a Category 3 cyclone is forecast to make landfall at Cairns, DON’T ASSUME that because the cyclone is further away that winds will be of a lesser severity to those you experienced during cyclone Larry.
Finally, despite the amazing technology about these days, cyclone forecasting is still far from perfect. Making it more of a challenge for our local weather bureau is the fact that Cyclones around Australia move more erratically than those from anywhere else in the world. And even when there is a high degree of confidence in the forecast track, there is ALWAYS a level of uncertainty. So keep that in mind, and always prepare for the worst.
Anyways, that’s about all I can be bothered typing tonight. I might add a few more issues as I remember them or after having further conversations……….