Tropical cyclones, in addition to causing utter devastation to our man-made world, can also be thought of as a magnificent example of the power of nature and the planet’s regulatory systems in full swing. And, with post-tropical cyclone Helene tearing across the North Atlantic recently, it's time to take stock on just what is a tropical cyclone.
The tropical cyclone is one of a number of different mechanisms that help to maintain the heat balance of the planet. It does this by re-distributing energy in the ocean-atmosphere system. A tropical cyclone is not the same as a mid-latitude low pressure system, although it looks very similar. It is much smaller in area, contains no fronts, and forms much nearer to the Equator. It depends principally on heat energy from the sea surface to drive it, rather than energy from the upper airstream.
So, how does a tropical cyclone work, more or less? Over the tropics, the atmospheric pressure tends to be naturally low, with the warm surface air continually rising.
But instead of rising uniformly, it does so in small chunks or ‘air parcels’ which tend to give rise to local thunderstorms. If there happens to be some atmospheric anomaly over the surrounding area, for example, the pressure is abnormally low or there is a strong temperature gradient, then several of these little storms might start to cluster together. This will cause a local increase in the strength of the updraft.
If there is enough Coriolis force (which is the effect of the Earth’s rotation that makes fluids turn as they travel over the Earth’s surface), it is zero at the equator and increases towards the poles to allow the air to spiral as it rises, a tropical cyclone will begin to form. The system will only continue to grow if the right combination of conditions exists to feed it. For example:
1. The sea surface temperature remains above about 26° C;
2. The atmosphere has a high humidity;
3. The wind speed and direction are fairly constant with altitude;
4. The disturbance remains far enough from the Equator (about 5° at least) for the Coriolis force to continue to have enough effect to turn the rising air into a vortex.
If the storm develops into a full-blown cyclone, it will turn into a self-perpetuating ‘engine’, feeding itself with the heat energy of the underlying water.
As the air spirals upwards, it is forced to cool, which forces the water vapour contained within the air to condense, which releases enormous amounts energy. The condensing water vapour is what creates those huge cumulonimbus clouds and torrential rain that come with a tropical cyclone. The energy released from condensation is pumped back into the system, maintaining the updraft and sucking up yet more moist air from below in a giant feedback loop. Once started, the ‘engine’ will keep going as long as it remains over warm water and is able to use the warmth of that water for its fuel.
The fully developed tropical cyclone is a large cylindrical mass of cloud with a hole in the middle, called the eye. Air is spiralling upwards around the outside of the eye (the eye wall). At the surface, the winds are strong and blow cyclonically (anticlockwise in the northern hemisphere and clockwise in the southern hemisphere). In the very centre of the eye itself, the air is descending, not rising. This keeps the pressure in the centre slightly higher than its surroundings, and is what creates the cloud-free, dry and windless conditions found in the eye.
Tropical cyclones tend to form between about 5° and 30° latitude. The season for tropical cyclones runs from about June to November in the northern hemisphere and about November to April in the southern hemisphere.
Once the sustained wind speed on the surface reaches 64 kts, the system is officially called a hurricane, typhoon, severe cyclonic storm, severe tropical cyclone or just a tropical cyclone, depending on the local protocol.
Cover shot by Nic Aberdein.