Tropical cyclones, hurricanes and typhoons are different names for the same thing – a high-powered storm system that poses a threat to any community they come near.
Ask someone to draw what a tropical cyclone looks like and you’ll probably be handed a set of swirling lines. At its heart, this is a pretty accurate representation of these massive storm systems, of which around 80-100 occur annually in tropical regions near the equator.
But while it might seem as though there are many of these big storms every year, they actually require several environmental components to come together at the right time.
Like baking a cake with one essential ingredient missing, the absence of one of these critical factors may prevent a hurricane from forming.
Just add water
Water, as with any storm, is the foundation of any cyclone. It falls from masses of swirling clouds at incredible speed, but to do that, it needs to get into the sky.
But while water routinely cycles from land to sky right across the globe, water in the tropics has one important property – it’s warm. For a cyclone to form, ocean temperatures need to be at least 26.5°C/79°F, which is commonplace in the tropics. This heat provides the energy required to form and sustain the cyclone, and because it’s warmer, more water evaporates to create higher relative humidity and plenty of storm clouds.
Location, location, location
Cyclones tend to develop and move within a particular band of the planet, between latitudes of around 5-20° north and south of the equator although they can move towards 30°. Good news if you live in large parts of Indonesia and New Guinea, bad news for northern Australia, South Asia, the Americas and parts of Africa.
The reasons for this are linked to our water temperatures, they’re consistently warmer in this band because they draw in more heat from the sun at those latitudes. But there’s another, more forceful reason – the Coriolis Effect.
The world spins fastest at the centre of the Earth and slower at the poles and the Coriolis Effect describes the way the planet’s spin at different latitudes causes air to ‘deflect in different directions. Cyclones that eventually form are directed because of this force – cyclones south of the equator spin clockwise; hurricanes and typhoons in the northern hemisphere spin in the opposite direction.
Within 5 degrees of the equator, Coriolis force is too weak, meaning cyclones, quite literally, can’t be spun up.
“From about 5° to around 20° north and south of the equator, that’s where all the ingredients come together,” says Dean Narramore, senior meteorologist at the Bureau of Meteorology.
The spin cycle
We have warm water, we’re in the right spot, now we can get our cyclone moving.
Our ocean has warm air floating above it. This warm, moist air rises into the sky, causing pressure to drop at the bottom of the column. Colder, high-pressure air rushes in to replace it, heats up, and begins to rise again. In the sky, the air cools, descends, and leaves water to form clouds, this continuous cycle builds more of these huge clouds into a massive ‘tropical low’ (or ‘tropical storm’ in the US).
“You get a large cluster of thunderstorms that focus around a central point, that induces a low-pressure system,” says Narramore.
Easterly winds tend to push these tropical lows in a westward direction in the Pacific. As they spin, fuelled by our pressure gradient drawing high-pressure air in faster and faster, they gain speed. Once their wind speeds hit 34 knots or 63km/h, you’re clocking at cyclone speed.
The ‘eye’ of the cyclone is relatively still and calm. But the spinning air closer to the centre turns much faster than air on the cyclone’s outer fringe.
“They continue to deepen and feed, particularly if the environment is conducive for development – hot sea surface temperatures, low-level shear,” says Narramore.
Low or weak vertical wind shear is another important factor. Shear is like a gradient, when it’s weak, there is little change in wind speeds from the top to the bottom of the cyclone. Strong or high vertical wind shear creates a larger speed gradient, and faster wind speeds at the top of the storm can cause the cyclone to break apart.
Assuming all conditions are right, the cyclone will continue growing in size and speed by sucking in more warm air – effectively fuelling itself. The longer it stays over the ocean, the more energy it can draw in.
Pulling a handbrake on a fast-spinning storm
Sustaining a cyclone needs plenty of energy and a lot of components to come together.
Should a condition change, the cyclone will begin to collapse and dissipate.
Moving into a region of cooler ocean temperatures effectively cuts the fuel line to the storm. It will dissipate as it moves towards the poles, for example, and once it hits land, it runs out of its energy source altogether.
“They’ll just keep developing and deepening while the atmosphere is conducive for it,” says Narramore.
