Coast KZN

21 Jan 2021

What is a tropical cyclone?

Tamlyn Jolly (Zululand Observer) Picture: Storm Report SA/Facebook. Eloise is making her way down the Mozambican Channel

With tropical storm Eloise making her way down the Mozambican Channel to make landfall on Saturday, this weather phenomenon is on everyone’s lips as we wait in anticipation. But what is a tropical cyclone and why is it called this?

Depending on its location and strength, a tropical cyclone is referred to by different names, namely hurricane (Atlantic and northeast Pacific oceans), typhoon (northwestern Pacific Ocean), tropical storm, cyclonic storm (south Pacific or Indian oceans), tropical depression or just a cyclone.

‘Tropical’ refers to the geographical origin of these systems, which form almost exclusively over tropical seas. Tropical cyclones are ranked on one of five tropical cyclone intensity scales according to their maximum sustained winds and which tropical cyclone basin they are located in. Tropical cyclones that develop in the southern hemisphere are officially classified on one of two scales, which are both based on 10-minute sustained wind speeds.

The Australian tropical cyclone intensity scale is used to classify systems within the Australian or south Pacific cyclone basin while the scale used to classify systems in the southwest Indian Ocean (where South Africa is situated) is Meteo France.

According to the Meteo France classification index, a tropical disturbance is a non-frontal area of low pressure that has organised convection and definite cyclonic surface wind circulation. The system should be estimated to have wind speeds of less than 50 km/h.

A tropical depression or sub-tropical depression is when the system reaches wind speeds above 50 km/h. Should a tropical depression reach wind speeds of 65 km/h it will be classified as a moderate tropical storm and assigned a name. Should the named storm intensify further and reach wind speeds of 89 km/h, it will be classified as a severe tropical storm. This is what Eloise was classified as when she made landfall in Madagascar on Tuesday.

A severe tropical storm is designated as a tropical cyclone when it reaches wind speeds of 118 km. Should a tropical cyclone intensify further and reach wind speeds of 166 km/h, it will be classified as an intense tropical cyclone.

A very intense tropical cyclone is the highest category on the southwest Indian Ocean Tropical Cyclone scale, with winds of over 212km/h.

While Eloise is, as of this (Thursday) morning, a moderate tropical storm, she is expected to intensify to that of an intense tropical cyclone before making landfall in Mozambique on Saturday.