Tropical Cyclone Lola’s evolution from a deep depression near the Solomon Islands to a beast nearly as powerful as they come has shone a light on how meteorologists formally classify these systems. Photo / WeatherWatch
Tropical Cyclone Lola’s evolution from a deep depression near the Solomon Islands to a beast nearly as powerful as they come has shone a light on how meteorologists formally classify these systems. Photo / WeatherWatch
Category 1, then Category 5 – and now Category 4.
Tropical Cyclone Lola’s evolution from a deep depression near the Solomon Islands to a beast nearly as powerful as they come and tearing over Vanuatu has shone a light on how meteorologists formally classify these systems.
But first, it’s important to understand how tropical cyclones - sprawling, swirling areas of low pressure packing gale-force winds - actually build.
We can think of them as giant atmospheric heat engines, drawing moisture from the warm ocean as fuel, and generating enormous amounts of energy as clouds form.
Rotating thunderstorms form spiral rainbands around their centre where the strongest winds and heaviest rain are found – creating a wildly-destructive “eye wall” around the eye.
This formation sucks heat 15km or higher into the atmosphere, while drier, cooler air at the top of the atmosphere becomes the exhaust gas of the heat engine.
Tropical Cyclone Lola as shown by RealEarth at 5.10am on Wednesday, October 25, 2024. Image / RealEarth
Some of the cool air sinks into the low-pressure region at the centre – explaining why cyclones often have calmer eyes with light winds and often clear skies, tens of kilometres wide.
The rest of the cool air spirals outward, away from the cyclone centre, sinking in the regions between the rainbands.
As long as environmental conditions support this atmospheric heat engine, a tropical cyclone can maintain its structure and even intensify over several days.
While we know them as tropical cyclones in New Zealand’s wider neighbourhood – the southwest Pacific – such systems are called typhoons in South East Asia and China, and hurricanes in the North Atlantic, where they rotate in the opposite direction, or counter-clockwise.
Unlike many other storm types, tropical cyclones can generate extreme winds, with gusts of more than 90km/h near their centre - or faster than 280km/h in the most severe cases.
These winds can wreak devastation on buildings and turn airborne debris into potentially lethal missiles, while torrential levels of rainfall and tide-raising storm surges can combine to cause disastrous floods.
As at yesterday, when Lola was tearing toward Fiji, weather services ranked the system at Category 5.
The system began as Category 1, putting it in the band where it packed 10-minute sustained wind speeds of 63km/h to 87km/h, before progressing to Category 2, where those wind speeds increased to 89km/h to 117 km/h.
The remains of homes in Savusavu, on the south coast of Vanua Levu island, pictured after the category 5 cyclone Yasa tore through Fiji in December 2020. Photo / Tinnah Tagiteci
When it reached Category 3 strength, it became considered a “severe” cyclone, with wind speeds of 119km/h to 157km/h - and its current band, Category 4, came with speeds of 159–204 km/h.
At the very strongest form, Category 5, which Lola briefly reached this week before being downgraded to 4, wind speeds measured at least 200km/h.
Of an average 10 tropical cyclones that form in the Southwest Pacific basin each year, only a few ever reach category 4 strength.
Category 5 events in the region are rarer still, with only two dozen or so – and most recently Niran in 2021 and Yasa in 2020 – having reached that scale over the last four decades.
Vanuatu and New Caledonia typically experience the greatest activity, with an average of two or three named cyclones passing close to land each year.
An average one system tracks within 550km of New Zealand each November-to-April cyclone season, usually around February and March.
This map shows tropical cyclone risk for the 2023-24 season. Source / Niwa
To get down here, they have to make their way over much colder waters, while hitting strong upper-level winds as they move out of the tropics.
In the process, they go through what’s called “extra-tropical transition” - and thus become “ex-tropical cyclones”.
But this doesn’t mean they’ve weakened or been downgraded, but rather have morphed into a completely different type of weather system.
Under the right conditions, they can intensify and muster even lower pressures than they had before being re-classified – as this year’s Gabrielle showed.
