Saturday the 19 October 2013 was another good day for storm spotting in Pretoria. Like most days that eventually end with good storms, it started out warm to hot with a bit of a stiff breeze, followed by a build up of clouds from the SW and SSW. I first spotted the beginnings of the storm just after getting out of the shopping mall near where I live. I was greeted by the feature in the picture below that appeared to be a roll cloud. It certainly didn't look like a shelf cloud because it was separate from the main storm cloud (shelf clouds are usually attached to the parent storm) but moving in the same direction and at the same rate of speed. It was pretty clear that this was going to be a dramatic storm.
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| The roll cloud as it approached ahead of the storm from the west |
And this dramatic feature (the image below) only served to confirm that this was no ordinary storm indeed. Wall cloud, scud or some sort of a shelf cloud, I guess I'll never know. All I can do at this point is just speculate as to what is was. But whatever it was, this storm was definitely shaping up to be a very exciting one. This wall cloud like feature though still continues to boggle me. What was it? Was it a wall cloud? Was it a shelf cloud? Was it scud? That's a question I'll always ask myself.
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| Was it a wall cloud? Was it a shelf cloud? Was it scud? |
What I did find out though is that this storm was in fact part of squall line that moved over the north of Gauteng that evening. I always get excited whenever I hear that a squall line is on the way or is passing over because squall lines always seem to produce very exciting weather.
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| Beautiful storm clouds behind the roll cloud as it passes overhead. |
So what is a squall line?
A squall line is basically a line of thunderstorms that have a common lifting mechanism. These can take on a number of different forms, called bands. They generally develop ahead of or parallel to a cold front or dry line boundary.
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| A squall line in the central United States on 5 June 2008 (Courtesy of National Weather Service) |
The above squall line is a classic example of squall lines that occur in the US during the spring and summer months and how large they can get. This particular squall line, which occurred on 5 June 2008, stretched from southern Minnesota in the north to Texas in the south. Of course, squall lines don't always get that big. Some squall lines can be a few tens of kilometres long to up to a 1000km long, like the one in the above image.
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| A squall line viewed from the space shuttle over Florida |
Squall lines are known to almost always produce some form of severe weather. They often produce strong gusty winds, as a result of the strong gust fronts they produce as they move along either a cold front or dry line boundary. They also produce heavy rain and moderate size hail. They rarely produce tornadoes, although tornadoes can form if supercells are imbedded within the squall line.
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| Shelf cloud associated with a squall line moving over the Belgian countryside (Courtesy of Erwin Klein) |
Squall lines are common in South Africa during the spring and summer months, especially in the Highveld. One of the most spectacular squall lines ever photographed in the country moved over Pretoria on 23 October 2012. It was a classic squall line storm featuring the typical long broad shelf cloud that stretched from the Pretoria CBD all the way down to the suburbs of Pretoria East and probably further than that, as well as the strong gusty winds that are typically associated with squall lines. It was an absolutely spectacular sight.
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| The only photo I managed to get of the Pretoria squall line on the 23 October 2012 |
If you would like to read more about this amazing squall line, you can go to http://www.stormchasing.co.za/forums/weather-photographs/7583-storm-over-centurion-pretoria-23-october-2012.
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