Convection and explosive thunderstorm development

Convection. It's a term we hear very often in conversations about the weather, especially pertaining to storms. We hear it a lot in weather forecasts, storm warnings and we read about it in weather journals, books and other weather related publications. 

So what is convection? How does it happen? And what role does it play in the development of storms?

The term itself is a general term that has many broad meanings. https://www.vocabulary.com/dictionary/convection

I will be focusing on the weather related side of convection on this post as I am more knowledgeable on that type of convection. From a weather perspective, convection (or more specifically atmospheric convection) refers to the heating of the ground by the sun which in turn heats the air just above it. This then results in a pocket of warm air which rises up into the cooler air above. Upon mixing with the cooler air it condenses and forms clouds. It also release energy in the form of latent heat. The differences in temperature between the warm air and cooler air above leads to atmospheric instability which, if the conditions are just right, will cause the air to expand and lead to the development of cumulus clouds. If there is enough moisture and enough instability is maintained, continued convection will lead to the development of cumulonimbus clouds, the characteristic clouds known to produce thundery weather around the world.

https://www.nc-climate.ncsu.edu/edu/k12/.convection 

Cumulonimbus cloud with a classic anvil top that spreads out as convection currents that help to form it reach the tropopause, were the unstable troposphere below meets the stable stratosphere above. The tropopause acts like a ceiling, stopping the rising updraft from going further up, causing it to spread out horizontally as convection currents continue to push it up from the ground.

Cumulonimbus cloud with an overshooting top. An overshooting top occurs when the updrafts in a thunderstorm are so strong that they basically punch a hole in the tropopause and continue to rise sometimes up to 2km up into the stratosphere. This is an indication of strong convection currents that can often lead to the development of supercell thunderstorms.

Another commonly used term in weather circles is explosive convection or explosive development. This of course is not really a scientific term but one that is very popular. It refers generally to thunderstorms (not necessarily severe thunderstorms) that develop very quickly, often within a matter of just minutes. Often the vertical movement of the clouds as they rise high into the atmosphere can be visible to the naked eye, giving one an idea of just how strong the convection currents and updrafts are. But the best way to really get an idea of how spectacular an explosively developing thunderstorm is, is by watching it in a time lapse video.

 Thunderstorms, at least here in South Africa anyway, develop explosively when there is high heat and humidity resulting in a high level of instability and strong convection. A strong jet stream also helps. In the Highveld region, explosive thunderstorms also develop when these factors are combined with the presence of a dry line boundary or a cold front that provides the lifting mechanism needed for them to develop.

Meteosat 0-degree Airmass Product, showing convection over southern Africa that typically brings intense (and mercifully brief) storms. Image source: EUMETSAT.

    The satellite image above shows a typical spring and summer weather pattern in South Africa, with intense and explosive thunderstorm activity in the east of the country, while the western part of the country remains largely dry. Taken on the 30 October 2013, it shows intense thunderstorm activity covering the entire Gauteng province, most of Limpopo, Mpumalanga, KwaZulu Natal and small sections of the Northwest Province and the Free State. Such intense and explosive thunderstorm activity is typical for this part of South Africa at this time of the year. Warm moist air from central Africa is drawn south to South Africa where, coupled with the heating of the ground and other factors like the presence of a dry line boundary gives rise to the powerful convection currents that lead to the development of the explosive storms that are typical for this region of the country during the spring and summer.

Spring and Summer storm clouds

Let me stress first that everything on this blog is based on general knowledge as I am not a meteorologist. I do however make every effort ensure that the information I provide comes from credible sources. If any part of the information I provide is incorrect or inaccurate in any way, I will most certainly appreciate the correct information being provided by those with expert knowledge.

Cumulus Congestus. Largest of the cumulus family of clouds and the final cumulus stage before transition to cumulonimbus

Thunderstorms are in my book the most spectacular weather events on the planet. What other weather system on earth produces such a mix of awesome and sometimes violent weather that thunderstorms produce? In addition to the weather they produce, the clouds that produce them can be just as spectacular.

Near perfect classic cumulonimbus cloud that formed to the far east of Pretoria somewhere over Mpumalanga on 27 October 2013. Although it was never confirmed, it is possible that this storm may have been a supercell

A spectacular cumulonimbus cloud that developed explosively over the east of Johannesburg on 15 December 2013. You can get an idea of how explosive it was in the second of the three time lapse videos below 

From broad cauliflower like cumulus congestus to the massive, towering cumulonimbus clouds that reach high into the stratosphere, nothing beats summer storm clouds when it comes to their ability to inspire awe in those who witness them. In the spring and summer, warm moist air from tropical Africa moves down towards the northern and central parts of South Africa, producing the often spectacular thunderstorms that are a common feature here during that time of the year. That, coupled with the ground heating from the summer sun, creates the convection that's needed for the clouds to develop and eventually grow into the cumulus and cumulonimbus clouds that bring our thunderstorms.

A roll cloud that formed ahead of an approaching storm in Pretoria on the 20 October 2013

One of the things I really love about the clouds that create thunderstorms is the way they develop. From inception to dissipation, there is almost a magical surrealness in the way the thunderheads shoot vertically into the upper atmosphere and the top spreads out to form the characteristic anvil that is the very epitome of the cumulonimbus cloud. One of my favourite ways of capturing a thunderstorm as it develops is through time lapse photography. There's just something about watching the way a cumulus or cumulonimbus cloud just explosively shoot up into the sky and evolve and change shape as it reacts to the environment around it.

 

The time lapse videos above are three of my favourite because they really capture thunderstorms at their best. I love the third one just for its eerie beauty and the way the clouds at the base of the thunderstorm almost seem to swirl around each other. This storm later produced heavy torrential rain that led to some minor flash flooding in parts of Pretoria.

Shelf cloud approaching Moreleta Park in Pretoria on 8 October 2013

Shelf clouds are among the most spectacular features of spring and summer storms. Their ominous and menacing appearance strike fear into the hearts and minds of those who see them. They are often mistaken for wall clouds and people often fear a tornado might hit when they see them approaching. But fear not. Shelf clouds do not produce tornadoes as they are an indication of an outflow dominant storm, that is a storm that is dominated by downdrafts. Tornadoes and wall clouds mostly form in storms that are inflow dominant, that is where warm air is being sucked up into the usually rain free part of a storm. It is still wise though to take shelter if you see a shelf cloud approaching as it may indicate the presence of very strong winds, heavy rain and often large hail.  

Lightning in Pretoria on the 28 November 2013

 

And last but not least is of course lightning. For me personally this is the most spectacular of all thunderstorm features. Yes, even more than tornadoes. Powerful electrical discharges that light up the evening sky and are so hot (up to 30,000 degrees Celsius, five times hotter than the surface of the sun) that they cause the air to expand rapidly in the form of explosive shock waves we hear as thunder.

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