Ken Stewart, 14 February 2011; updated 20 February
Prior to the arrival of Tropical Cyclone Yasi, Premier Anna Bligh repeatedly warned Queenslanders that Yasi would be very severe and dangerous, and urged people in its path to evacuate, repeating warnings from the Bureau of Meteorology:
“This impact is likely to be more life-threatening than any experienced during recent generations,” the Bureau of Meteorology said this morning. (Feb 02 2011)
People heeded the warnings and although there was much damage, only one life was lost.
However, the reality of Yasi’s impact was quite different from the prediction. Using data from Yasi’s approach to Willis Island, I calculated estimated maximum speed of 220 to 240 km/hr at landfall. This was borne out by scientific assessment of damage in the area:
“The CTS team leader in the field, Dr Geoff Boughton, said that the analysis of damage to simple structures throughout the region indicated that the wind speeds on the ground in Tropical Cyclone Yasi were less than those expected in a Category 5 event.
Using techniques developed and refined in previous cyclones, the CTS team has formed a preliminary view that the maximum wind speed in Cyclone Yasi was about the same as that in Tropical Cyclone Larry.
“We estimate that the gust wind speeds in some of the most affected areas were about 220 km/h” Dr Boughton said.”
In other words Yasi was a strong Cat 3 or very low Cat 4, not Cat 5. There is no evidence for winds of 290km/hr . The danger from Cyclone Yasi was exaggerated and over-hyped.
What’s the big deal? you may ask. Lives were saved.
True. Hyping the danger was a good short term tactic and worked. However, this is not a good long term strategy in our fight to defend ourselves from nature’s extremes.
The people of North Queensland may well say, “Well if that was 290 km/hr winds, the damage was bad, but not too bad. We’ll be right next time.”
But the damage from a cyclone with real 290 km/hr winds will be many times worse than that caused by Yasi.
This is from BOM’s FAQs about cyclones http://www.bom.gov.au/cyclone/faq/index.shtml
Or to rephrase the question: Would a cyclone with wind gusts of 280 km/h cause twice the damage of a similar sized cyclone with wind gusts of 140 km/h? No – it would cause hundreds of times more damage.
As wind speed increases the power of the wind to do damage increases exponentially. Hence a category 5 severe tropical cyclone (with wind gusts > 280 km/h) has the potential to do around 250 times the damage of a Category 3 severe tropical cyclone (with wind gusts of 165 km/h). This underscores the importance of the category system.
Graphical representation of the variation of damage with wind speed.
I have marked in the maximum gusts from Yasi (61 m/sec) and the predicted strength (80.5 m/sec). Wind increases 33%, damage increases by about 1100%! (Don’t ask me how they get “250 times” the damage for 165-280km/hr increase!)
This is serious! With the strong La Nina, there are likely to be more cyclones this season. People lulled into a false sense of security by believing they have survived a Category 5 cyclone may be killed.
Here’s a graph of annual SOI values since 1910. Notice the dip in the late 1970s and the recent rise. If this trend continues, we may be entering a phase of more La Ninas and fewer El Ninos in coming years- meaning more cyclones.
What Australia-wide strategy is needed?
- Educate the public about the realities of La Ninas, cyclones, floods, droughts, fires.
- Mandate storm shutters for windows and household storm shelters in cyclone prone areas.
- Have highly visible marks on every power pole and public structure showing highest known storm tide or flood height.
- Mandate flood height information to be part of every real estate transaction and on every rate notice.
- And in all disaster events, tell the truth, don’t exaggerate. The media will do that anyway. Give factual details in a calm clear manner.
Hype is dangerous.