WINDS OF CHANGE: What's the deal with storms in Southern Africa?
What's the difference between depressions, tropical storms and cyclones, and why do certain regions experience such varying weather conditions?
JOHANNESBURG - In recent years South Africa has had to contend with Tropical Cyclone Dineo and Eloise but before them, there was Domoina, Hudah, Nadia, Bonita and Irina, to name just a few.
WHAT EXACTLY IS A TROPICAL STORM?
According to the South African Weather Service, a tropical cyclone is a “relatively small, intensely developed low-pressure cell that usually occurs over warm oceans”.
The diameter of a cyclone can range between 200km and 2,000km. They are characterised by a warm centre, with very steep pressure gradients and strong cyclonic winds near the Earth's surface.
DEPRESSIONS, STORMS AND CYCLONES
Storms with the above characteristics and a maximum wind speed of less than 60 km/h are called tropical depressions.
When the maximum wind speed ranges between 60 and 110km/h, they are tropical storms, and when the maximum wind speed exceeds 110km/h, they are called tropical cyclones.
In the North Atlantic and eastern North Pacific regions, such events are known as a hurricane. In the western north Pacific, the term used is typhoon.
HOW IS A TROPICAL STORM FORMED?
NASA likens tropical storms to “giant engines” that use warm, moist air as fuel.
This is why tropical storms form only over warm ocean waters near the equator.
The warm, moist air over the ocean rises upward from near the surface.
This causes an area of lower pressure below and air from surrounding areas with higher air pressure to push into the low-pressure area. Then that so-called "new" air becomes warm and moist and rises, too.
While the warm air continues to rise, the surrounding air swirls in to take its place. As the warmed, moist air rises and cools off, the water in the air forms clouds.
The entire system of clouds and wind spins and grows, fed by the ocean's heat and water, it evaporates from the surface.
Storms that form north of the equator spin counter-clockwise and those that form south of the equator spin clockwise. The Earth's rotation on its axis is what causes the difference.
As the storm rotates faster, an “eye” forms in the centRE. It is said to be very calm and clear in the eye, with very low air pressure. The higher pressure air from above flows down into the eye.
Cyclones usually weaken when they hit land as they are no longer being "fed" by the energy from the warm ocean waters. And they may move inland, dumping a lot of rain and causing lots of wind damage before they die out completely.
WHAT'S IN A NAME?
In the West Indies for several hundred years, storms they were named after the particular saint’s day on which the hurricane occurred.
In the United States, meteorologists gave tropical storms women’s names.
Satellites were first used by the military, and it is said that Air Force and Navy meteorologists who plotted the movements of storms named these storms after their wives and girlfriends.
But the official way to name cyclones was to refer to their latitude-longitude position, but this became confusing when the storms moved.
Naming tropical cyclones solely after women came to an end in 1978 when men's and women's names were included in the Eastern North Pacific storm lists.
In 1979 all names were included in lists for the Atlantic and Gulf of Mexico.
Lists of names are compiled and maintained by various meteorological committees. Each list has a name for each letter of the alphabet starting with A and ending with Z. A name on the list will be replaced if a storm given this name causes extensive damage and loss of life.
Names are suggested and pre-determined per season.
Below are the tropical cyclone names and the country that suggested the name for the 2020/21 season for the South-West Indian Ocean.
Kanga South Africa
Urilia South Africa
A short history of tropical storms in Southern Africa:
TROPICAL STORM DOMOINA (1984)
Domoina hit South Africa in 1984 in the St Lucia area in South Africa. It struck in Southern Mozambique and crossed westwards into Eswatini. Mpumalanga and KwaZulu-Natal were hit hard and declared disaster zones. The fourth-named storm of the season, Domoina developed on 16 January off the north-east coast of Madagascar.
Cyclone Hudah made landfall along the east coast of Madagascar between Mahanoro and Mananjary in 2000. The northernmost area of Madagascar was the most affected region. It affected the South-East of Africa.
Cyclone Nadia, category 1, started north of Madagascar and crossed Nampula province and moved south through Zambézia and Sofala, killing 240 people.
A category 1 cyclone, it crossed Madagascar, hit Zambézia, and crossed Zimbabwe and Zambia and became the first recorded Indian Ocean cyclone to reach the Atlantic Ocean.
Tropical Storm Irina was a large tropical cyclone that brought gusty winds and torrential rain across Madagascar, Mozambique, and South Africa.