As South Africa lives through one of its hottest and driest summers in years people are scanning the heavens for harbingers of rain. And as we saw again on 16 November last year, thunderstorms can come up suddenly and cause havoc. Large swaths of the northern parts of the country were subjected to severe hailstorms on that day and hailstones the size of tennis balls inflicted massive damage in Limpopo.
Dr Liesl Dyson of University of Pretoria (UP)’s Department of Geography, Geoinformatics and Metereology went on record, however, to the effect that hailstones of unprecedented size are not necessarily explained by heat or drought.
“The truth is that surface temperatures have to be high to form a substantive condition for the occurrence of hail, but surface moisture is a further requirement.”
Dr Dyson says hail mainly occurs in the summer-rainfall regions of South Africa, usually early in the season from October to December. “The atmosphere turns more tropical later in the season, and although good rains might still fall the chances of hail will be minimal.”
Air cools as it rises, and the low-pressure system where hail is formed is called the freeze zone because the temperature at this level is below freezing. The altitude of this freeze zone must be fairly close to the earth’s surface for hail to form.
Another condition that favours hail formation is collision between hot air rising from the earth’s surface and the resultant flow of cold air flowing across the land from the sea. “Heavy thunderstorms that may be accompanied by hail will tend to develop along the line where warmer and cooler airflows meet.”
Air heated by the hot surface of the earth will begin to rise, causing strong updraughts. During a thunderstorm air bubbles containing moisture will be borne further upwards where the cooler air will cause condensation and the moisture in the ‘air pocket’ will change phase and become water. When the condensate moves through the freeze zone it will freeze.
Dr Dyson explains that rising and falling in this manner may contiunue for some time. The ice particles formed at higher altitudes will impact others and form clusters that will gradually overcome the force of the updraught and then fall to earth as hail.
“Frequently the ice particle melts as it leaves the cloud and enters the warm atmosphere, in which case it merely precipitates as rain. So, if you come across a hailstone the size of a tennis ball on earth it must have been quite a whopper when it came down from the cloud.”
Dr Dyson says there aren’t many signs that a hailstorm is on its way, although mammatocumulus clouds (see photo) could be an indication of a severe hailstorm in the offing. It may also be advisable, therefore, to heed warnings from the Weather Service.
Dr Dyson classifies a thunderstorm as a relatively minor weather phenomenon, a large one being approximately 5 km across, compared to a tropical cyclone, which can measure thousands of kilometres across.
Dr Dyson warns, however, that any thunderstorm is potentially dangerous because of attendant phenomena such as hail, lightning, heavy rainfall and winds gusting at gale force.
Statistically lightning strikes claim between 1,5 and 8,8 deaths per million of the world’s population each year. The toll is higher in South Africa, and especially in the eastern Highveld where air pollution could be a possible cause of the higher incidence of lightning strikes. People should therefore seek shelter when lightning strikes are in evidence.
As regards the current dry weather conditions Dr Dyson points out that simlar droughts have also been caused in Gauteng by the El Niño phenomenon in 1982-83 and 1991-92. Although the weather conditions caused by this phenomenon are worrying they are not unduly portentous.
See related photo on the right sidebar.
Dr Liesl Dyson
January 4, 2016
Professor Liesl Dyson studied Meteorology at the University of Pretoria (UP), which is the only university in South Africa that offers a degree in Meteorology that complies with the World Meteorology Organisation’s training standards. Prof Dyson has been doing research at UP for 25 years, and says “the beauty of clouds inspired me to study meteorology”.
As an associate professor in Meteorology in the Department of Geography, Geoinformatics and Meteorology, research is part of what she does. The research that she and her students are involved in deals with understanding the character of severe weather phenomena, investigating the development mechanisms of these phenomena and finding ways to better predict these events. As for how her research contributes to the betterment of the world, she says that timeous warnings of severe weather could save lives and property.
Many of the postgraduate students who do research under her supervision are employed at the South African Weather Service.
“Together, we identify which weather phenomena needs further investigation, then set about improving the understanding and forecasting of these,” Prof Dyson explains. “At the moment, there is special emphasis on severe thunderstorms. We investigate thunderstorms by using radar and state-of-the-art numerical weather prediction products. However, other phenomena such as sand/dust storms and snow are also included in our research.”
Prof Dyson collaborates with the Natural Hazard Centre, which statistically analyses the hail data she generates by using numerical weather prediction data and a hail model. The result of this collaboration determines the risk of hail over South Africa.
A highlight for her was being awarded the Water Research Commission’s Knowledge Tree Innovation Award in 2019 for a project entitled ‘The impact of the predictability of continental tropical lows on hydrological modelling: Current state and future projections’. This award celebrates outstanding design, process and product development of innovative technologies. The research investigated and named a specific type of tropical weather system over southern Africa, which they called Africánes. The system is a tropical cyclone-like low-pressure system that develops over Africa and causes very heavy rainfall over the sub-continent.
Two of the master’s students working under her supervision received the prestigious Society of South African Geographers Master’s Bronze Medal, which is awarded to the best geography MSc completed by research dissertation within a given year in South Africa. Christina Liesker received the award in 2022 for her work on supercell identification using weather radar and Markus Geldenhuys in 2019 for his work on the dangerous mountain wave phenomena over the Eastern Cape.
Prof Dyson’s role model in meteorology is Dr Jan Taljaard.
“I had the privilege of working with him for a short while when I was a young forecaster at the South African Weather Bureau. I admire him so much because he was an expert in real weather. It was all about what was going on outside and he instilled in me a hunger to do the same. He authored several papers about synoptic circulation over South Africa, and even though he has long since passed away, these documents are still my go-to papers when I do my own research. I discover new insights every now and again.”
Prof Dyson hopes to play a small part in helping weather forecasters in South Africa to be enthusiastic about the weather, be excellent in their jobs and have the knowledge through training and research to issue forecasts of high quality.
Research shows that in a changing climate, severe weather events are increasing. On short time scales, weather forecasters are responsible for issuing warnings for these events. Prof Dyson’s research matters because it aims to help forecasters to issue such warnings.
She says many young people are interested in the weather because they see and experience the effect of climate change and want to get involved in helping to address arising issues. Others are simply fascinated by tropical cyclones (hurricanes) and tornadoes, and by clouds. She advises school learners that in order to study meteorology, they need to take mathematics and natural science as subjects at school.
“We use Newton’s second law and the laws of thermodynamics to model the atmosphere, and learners need the mathematical tools to do that.”
Prof Dyson loves gardening in her spare time and enjoys outdoors activities with her dogs.
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