Hailstorms

Hail is one of the most destructive natural forces on the planet.These rock hard balls of ice cause billions of Rands worth of damage every year in South Africa alone. And just over a week ago, the residents of Gauteng got to experience first hand just how destructive hail can be. Both Johannesburg and Pretoria were on Thursday the 28 November 2013 pummeled by the most destructive hailstorms to strike both cities in years. In some areas the hailstones were reported to be up to baseball size. In Pretoria, several areas in the townships of Mamelodi and Soshanguve in the north of the city were pummeled by tennis ball to baseball size hail that shattered and smashed windows, punched holes in roofs, dented cars and smashed their windscreens. In Johannesburg, parts of the East and West Rands were also hit by massive hailstorms, causing a great deal of damage. In addition to the hail, damage was also caused by strong winds as well as some localised flooding. But it was the hailstorms that were the main topic of conversation that day.

The leading edge of one of the supercell thunderstorms that pummeled Gauteng with massive hail on the 28 November 2013 passing over Moreleta Park in Pretoria

 So what is a hailstorm, and how does it form? In simple terms, a hailstorm is a storm that is characterised by hail. Now obviously that is not a good answer, and not a good question either. The appropriate question would be, what is hail? Well, hail is a form of precipitation charaterised by often large and very hard clumps of ice that form within the updraft of a thunderstorm. At first glance, we may tend to think that hailstones are made up of a single solid ball of ice. That is not the case. Instead of a single solid ball of ice, hailstones are made up of several layers of ice forming on top of each other as it is carried up and down in the downdrafts and updrafts of thunderstorms. How big the hail gets is largely dependent on how strong the updrafts in the thunderstorms are.   

Hailstones cover the ground in Mamelodi in the north of Pretoria following the hailstorms on the 28 November 2013. Courtesy of Victor Mbinga/iWitness

The two videos below were shot in Pretoria and can be viewed on the following links http://www.youtube.com/watch?v=hIxHRS4T3D0 and http://www.youtube.com/watch?v=7jKnks4i4es respectively. Neither are my own videos, but I included them just to illustrate how ferocious the hailstorms of the 28 November 2013 were. 

 

The diagram below is a clear but simple illustration of how hail forms. As most of us are already aware, a thunderstorm is composed of an updraft and a downdraft. An updraft is a pocket of warm air which, being lighter, rises up into a thunderstorm, while a downdraft is colder and therefore heavier air falling down to the ground. The interaction between these two different components of thunderstorms lies at the heart of hail formation. As raindrops are sucked up into the storm by the powerful updrafts, they reach the freezing level where they, well....., freeze. The updrafts within the storm prevents them from falling to the ground until they reach a size and weight that makes them too heavy to stay aloft. As they are circulated up and down within the updrafts and downdrafts of the storm, they build up layer upon layer of ice until such a time that they get too heavy to be held up in the clouds, at which point they fall to the ground. How big they get is of course determined by how many cycles they go through in the storm clouds before they eventually fall to the ground. More cycles, more layers, bigger hailstones. The number of cycles is in itself determined by how strong the updrafts within the storm are. And generally, the strongest updrafts tend to develop in the strongest storms. That is why supercell thunderstorms tend to produce the biggest hailstones.  

A diagram of how hail forms in a thunderstorm. Courtesy of http://www.nc-climate.ncsu.edu/edu/k12/.SevereWeather

 The photograph below shows exactly why hail is so destructive. Aside from the jagged edges on the outer perimeter of the hail, the layers that make up hailstones are clearly visible. This particular hailstone, which appears to be at least baseball size, certainly went through quite a number of cycles before it fell to the ground, which means it most likely formed in a very powerful storm indeed.

A large hailstone cut in half to show the different layers it is made up of. http://www.nc-climate.ncsu.edu/edu/k12/.SevereWeather

 Although they don't quite match the hailstone in the above image, the hailstones in the image below fell in Krugersdorp (a well known hail hotspot) in the west of Johannesburg. They appear to be about golf ball size, which still makes them very dangerous. Hail this size is capable of denting cars and smashing windows.

Golf ball size hail that fell in Krugersdorp, west of Johannesburg on the 28 November 2013. Courtesy of Gavin Carter/iWitness

Due to its location and climatic conditions, South Africa is by no means immune to hailstorms. In fact, as the storms of the 28 November 2013 demonstrated, hail is a very common feature of our weather during the spring and summer months. Most hailstorms are concentrated in the summer rainfall belt of the country. Which is not surprising since that is where most of the country's thunderstorm activity is concentrated. In South Africa though, as the map below indicates, hail frequency seems to be associated more with altitude, with the high lying regions of the Drakensberg mountains in Kwazulu-Natal, the Free State, the Eastern Cape and practically the whole of Lesotho showing the highest frequency of hail occurrence. http://planet.uwc.ac.za/NISL/Invasives/Assignments/GARP/atlas/atlas_252t.htm 

Hail day frequency map of South Africa (School of Bioresourses Engineering and Envonmental Hydrology University of Natal Pietermaritzburg South Africa)

Hail producing storms are also known to produce some spectacular cloud formations. These can take many forms, such as mammatus clouds like the ones in the photo below. These were at the back end of the storm that pummeled Pretoria on the 28 November. They were quite beautiful to watch. An example of how beautiful these storms can be while being so destructive at the same time.

Mammatus clouds at the tail end of a supercell thunderstorm in Pretoria on the 28 November 2013