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What causes large damaging hail to form?

Large hail stones fell in Krugersdorp on Monday 9 October 2017.

Large hail stones fell in Krugersdorp on Monday 9 October 2017.

On Monday, a line of severe and destructive thunderstorms went through Gauteng, resulting in large hail stones, flooding and ‘mini-tornadoes’ also known as funnel clouds. So what causes a short lived thunderstorm to develop into a severe thunderstorm with hail?

VIDEO - Hail stones damaging a car windscreen in Gauteng on Monday 9 October 2017

In order for large hail stones to form, the environment firstly has to be conducive to thunderstorm development. A conducive environment has three key ingredients; moisture, instability and a trigger.

Moisture is a measure of humidity and is needed for a cloud to form, instability means that once a parcel of air is forced to rise; usually due to daytime heating of the earths surface or instability in the upper atmosphere it will continue to rise on its own accord into the upper atmosphere to form cumulonimbus clouds. The trigger is what causes the initial parcel of air to rise – near the surface this is usually due to daytime heating caused by the sun, converging wind patterns, a dryline or topography. Another trigger could be a sharp upper atmospheric trough – in simple terms this ‘sucks the air in the lower atmosphere upwards’ , like a vacuum cleaner.

So now we have the ingredients for a thunderstorm to develop, what causes it to become severe with large hail, strong winds and flash flooding? The answer to this is wind shear and a very unstable atmosphere – these two factors allow the thunderstorm to build over a few hours and therefore become very powerful.

Instability is measured in many ways but the most common way of measuring it is using CAPE (Convective Available Potential Energy). For a thunderstorm to become severe, large amounts of CAPE are required.

A simple way to explain wind shear is air coming into the storm in one direction and exiting in another.

Damage due to severe thunderstorms in Krugersdorp on Monday 9 October 2017.

Damage due to severe thunderstorms in Krugersdorp on Monday 9 October 2017.

Wind shear allows the thunderstorm to become bigger and bigger without collapsing on itself. If there is no wind shear then the updraughts and downdraughts end up counteracting each other and will eventually cause the thunderstorm to weaken after about 30-60min.

If there is good wind shear and high levels of CAPE then this allows the thunderstorm to last a few hours and the water droplets to be tossed high in the atmosphere over and over again, eventually the water droplets turn to hail and the longer they are tossed up and down the larger the hail will become.

Eventually the weight of the hail becomes too much and large hail stones are dropped to the earths surface.

AfricaWeather, Smarter Weather Insights.

2 Responses to “What causes large damaging hail to form?”

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  1. [...] For thunderstorm activity to develop the atmosphere has to contain three key ingredients, moisture, instability and a trigger. This was discussed in detail in a previous blog – What causes large damaging hail to form. [...]


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