What is a Category 5 cyclone and how strong are they?
Cyclone Ilsa, an intense storm system swirling off Western Australia, is now expected to become a Category 5 cyclone, the strongest category in the rating system.
So what does this mean, and just how destructive will it get?
Here is everything we know about the rating system.
READ MORE: The devastating natural disaster that's going to get more intense
The damage from Darwin's Cyclone Tracy in 1974 was estimated to be $200 million. That was only a Category Four storm. Fairfax Media archives
How are cyclones categorised?
A numerical scale from one to five is used to rate the destructiveness of cyclones, with five being the strongest.
The intensity ratings are based on maximum wind gusts associated with the cyclone.
These usually occur around the eye of the storm.
The category system for cyclones is measured according to maximum wind gusts. Craig Abraham/The Age
What is the strongest cyclone category?
Category 5 is the strongest cyclone category.
How strong is a Category 5 cyclone?
Andrea Pearce, senior meteorologist at the Bureau of Meteorology (BoM), said a Category 5 cyclone has "estimated gusts of 280km/h and above".
Isla is expected to make landfall later tonight with wind gusts in excess of 280km/h.
"The very destructive core of Ilsa, with extreme gusts up to 285 km/h is expected to impact the area," BoM said.
Rapidly-developing Tropical Cyclone Ilsa is expected to become a Category 4 storm by the time it makes landfall in Western Australia. Supplied
Category 5 cyclones history in Australia
Cyclone Darian, which formed in December well off Australia's mainland was the last Category 5 cyclone to fall in the country's area of responsibility.
However, Cyclone Lawrence in 2009 was the last category five to make landfall.
It hit Eighty Mile Beach in WA on December 21.
"Mandora recorded a wind gust to 211km/h during landfall, but it likely produced gusts of 285km/h at its peak," Weatherzone said.
Worst cyclones in Australian history
The deadliest tropical cyclone in Australian history was Cyclone Mahina, another Category 5.
It struck Bathurst Bay in Far North Queensland on March 5, 1899 - it killed an estimated 410 people when it scuttled a sheltering pearling fleet.
Mahina is the deadliest cyclone in Australia's history. Supplied
A powerful La Nina fuelled Cyclone Yasi which devastated north Queensland. Craig Abraham
Cyclone Yasi, which hit Far North Queensland coast in February 2011, was one of the largest storms in recent history.
The slow-moving Category 5 system resulted in thousands being evacuated from Cairns.
Banana and sugar cane crops were flattened by the 285km/h wind gusts.
It was estimated to cause $3.6 billion in damage – the most costly cyclone in Australia to date.
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