Cyclone categorisation is a complicated business. There are multiple different names and the rating systems are different all over the world.

Cyclone Marcus hit Darwin as a category two and Nora hit Cape York as a category three, but what does that actually mean for damage on the ground?

Associate Professor Liz Ritchie of the University of New South Wales studies tropical cyclones, hurricanes and typhoons — different names for the same thing.

"[Tropical cyclones] are categorised by how intense their most intense winds are. The most intense winds usually lie very close to the eye, or the wall just outside of the eye," Dr Ritchie said.

"They categorise it [the wind speeds] into chunks, through category one, two, three, four, five."

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But the categories are not the same all over the world. A category one tropical cyclone in Australia is not the same as a category one hurricane in the US, which is not necessarily the same as a typhoon crashing into the Philippines.

Most parts of the world, including Australia, calculate the sustained wind speed by taking the mean wind speed measured over 10 minutes.

But in the US, forecasters only use one minute means and in the north Indian Ocean they use three minutes.

An Australian category 1 cyclone is less severe than a category 1 hurricane in the US. ( Supplied: Bureau of Meteorology )

So there are discrepancies across the globe but that is not the only issue.

What's wrong with the system?

The current system has been working all over the world for many years but it only takes into account the maximum winds, which was a limitation according to Dr Ritchie.

"It doesn't capture how big the cyclone is and that's important because the damaging winds can extend quite a long way from the centre," she said.

One of Darwin's main streets the morning after the category 4 Cyclone Tracy hit. ( Supplied: Baz Ledwidge )

Dr Ritchie said the extent of the damaging winds also drove the storm surge, the high waves associated with cyclones that could bring about flooding.

How quickly the cyclone was travelling was also important.

"The faster [tropical cyclones] move the less damage in one area, but the greater the extent of the damage might be, but the less total damage in one area."

How then to accurately convey the total damage a cyclone can do?

"There are definitely centres who are trying to capture those parameters in other ways, but there is no one catch-it-all parameter out there," she said.

Cyclone Damage Potential

One team which has developed an alternative method of describing cyclone damage is the US National Centre for Atmospheric Research.

Dr Greg Holland says there is better way to measure the potential for cyclone damage. ( ABC Weather: Kate Doyle )

Recently retired Australian scientist Greg Holland was one of the team who developed a rating system called the Cyclone Damage Potential (CDP), which takes into account the speed and size of the cyclone.

"So we've really taken just this one number and just added a couple of other numbers to it, that actually gives you a much better estimate of the local damage," Dr Holland said.

The CDP rates cyclones on a scale from zero to 10, where ten is the most damage.

The formula for calculating the original Cyclone Damage Potential (CDP). ( Supplied: Greg Holland )

As well as taking into account the speed and the size this original index is global, so you can use it to directly compare a tropical cyclone in Australia to a hurricane in the US.

The vulnerability factor

The team has gone beyond the original system and developed another which takes into account how much damage a storm would do to the infrastructure in an individual area.

"If a tropical cyclone's going through a thatched roof village on the side of a beach, you can have a lot of damage at a lot lower level than if it's going through a very well-engineered city," Dr Holland said.

So this system is different for each place but is a better guide of how much damage will be done.

This scale also has a range of zero to 10, where zero sustains very little damage and but a 10 means severe damage.

A house, destroyed, with debris lying everywhere in Tully after Cyclone Yasi hit in February 2011. ( Supplied: Dave Hunt, AAP )

Dr Holland has a few examples of past cyclones' CDP with vulnerability factor:

Cyclone Yasi, North Queensland 2011, was an Australian category 5 and had a CDP of 10

Cyclone Yasi, North Queensland 2011, was an Australian category 5 and had a CDP of 10 Cyclone Tracy, Darwin 1974, was a category 4 but also had a CDP of 10. Tracy travelled very slowly

Cyclone Tracy, Darwin 1974, was a category 4 but also had a CDP of 10. Tracy travelled very slowly Cyclone Larry, North Queensland 2006, was a category 5 and had a CDP of 4

Cyclone Larry, North Queensland 2006, was a category 5 and had a CDP of 4 Cyclone Marcus, which hit Darwin last week as a Category 2, is complicated because it spent a fair bit of time over land beforehand, but an approximate value for CDP would have it around 1–2.

The CDP will be used for forecasts in the United States Gulf Coast next year, but Dr Holland did not think the cyclone damage potential should replace the old categories altogether.

"In the case of the category system, it's well known, it's well established and people understand it," he said.

"I would use this as an adjunct saying 'and here is some extra information that you can compare that to'.