Weather bureau forecasters declare a cyclone when its average sustained wind speed exceeds 63km/h and it is declared severe when speeds pass 118km/h.
Every cyclone has an eye or centre which can vary from 10km to 100km depending on the storm’s severity. The eye has relatively clear skies (you can see the stars at night!)
The eye is surrounded by a dense ring of cloud about 16km high known as the eye wall which is the belt of stongest winds and heaviest rainfall.
After the eye passes the other side of the cyclone hits with the wind blowing in the opposite direction. Southern hemisphere cyclones spin clockwise.
Coming in from the sea they are accompanied by massive storm surges.
Cyclone Wanda a small cyclone in early 1975 caused Brisbane’s devastating floods then.
There have been more than 207 cyclone impacts along the east coast since 1858.
Climate change may be playing a part. Ocean tempertures around northern Australia are about 1.5C above pre-1970s levels and will produce more intense La Ninas even if the frequency remains the same. Warmer oceans produce warmer air above and this can hold far more moisture to become rain and cyclonic energy.
Cyclones are steered by upper-level influences by systems in the mid-levels to upper levels of the atmosphere. Tropical cyclones can form very quickly.
The Southern Oscillation Index (SOI)gives some advance warning of a heavy cyclone season ahead. It is about atmospheric pressure. In the summer of 2010-11 it has been consistently extremely high (mid to high 20s historically associated with floods and cyclones).
About twice as many cyclones form during moisture-bearing La Nina conditions. Bureau of Meteorology forecast in October was for 6 or 7 cyclones this season because of La Nina but this is a general forecast only. The weather cycles irregularly between El Nino and La Nina. El Ninos tend to occur roughly every 3 to 8 years and typically last 12 – 18 months. The rest of the time La Nina or neutral conditions dominate. These conditions stretch thousands of kilometres westward across the equatorial Pacific Ocean from the South American coastline.
The Madden-Julian oscillation index (MJO) is the largest element of the variability in the tropical atmosphere. It is not dissimilar to the La Nina effect in that it is caused by a combination of winds circulating around the atmosphere and sea temperatures but it is guided by extreme heat and runs from east to west in proximity to the equator. They start in the Indian Ocean. The pattern for an MJO (the active phase) is about once amonth.
MJOs are quite separate from La Nina but when the two of them are both happening cyclones ‘are on’. For example an MJO and La Nina were both happening in mid-January and resulted in Cyclones Anthony and Yasi.
The other long term weather cycle to take account of is the cold Pacific decadal oscillation.
Queensland’s cyclone season (November to April)is the time of year when conditions are perfect for the biggest storms to form. However most form in February and March. On average about five cyclones a year affect Queensland.
How cyclones are rated:
Category 1: Winds of less than 125km/h. Minimal house damage.
Category 2: 125-164km/h destructive winds. Minor house damage. Significant damage to trees and caravans. Heavy damage to crops.
Category 3: Very destructive winds 165-224km/h. Roof and structural damage. Power failure likely.
Category 4: Very destructive winds of 225-279km/h. Significant roof and structural damage. Dangerous airborne debris. Widespread power failures.
Category 5: Severe cyclone. Winds of more than 280 km/h. Extremely destructive and dnagerous with widespread destruction.