Don’t confuse Ace Ventura with the Venturi effect.
I am lucky to be doing a presentation in Paarl next week and not this past weekend.
What a scary wind that was.
Most have seen video clips on social media of a trailer and caravan being blown off the bridge just past the Huguenot tunnel.
I think we all ask ourselves how strong that wind was that it could cause so much destruction.
I can even hear many “experts” shouting that the wind never gets that strong in our neck of the woods.
If I told you it was not so much the wind speed which caused the toppling of the vehicles, but more the topography and wind direction, you would be a little confused.
To explain it in the simplest of terms, imagine a parcel of air moving towards our country’s coastline.
Its speed is governed by the pressure gradient and the smooth surface of the sea does not slow it down.
When it hits the coast and moves inland, it slows due to the roughness of the land (buildings, trees and hills).
The wind in question blew from the southeast, travelling from the Agulhas coast area, where it gusted at 80km, to the Cape Fold Mountains, stretching from Swellendam to Worcester, where gusts had dropped to 50km/h.
The wind then veered to a more easterly direction along the mountainside, eventually forming a massive bottleneck in the Worcester “basin”, east of the Du Toitskloof Pass.
The Venturi effect then produced its magic and accelerated that wind to an astronomical speed capable of blowing over what looks like a 10-tonne truck’s enclosed trailer and a caravan.
By the time it hit Paarl it was less constrained and spread out over the area, by then having dropped from an unknown speed to 105km/h.
To aggravate matters, as you exit the tunnel on the Paarl side of the pass, the road gently curves to the left and then immediately into a sharper right-hand curve atop the bridge where the vehicles were blown off.
In so doing, it turns vehicles side-on into the wind, making them vulnerable to toppling, especially those with some height.
The original builders of the Du Toitskloof Pass traversed the tributary of the Krom River higher up, closer to the mountain, using stormwater pipes under the road, opposed to the Huguenot tunnel builders who traversed the river with a relatively high bridge.
Incidentally, our Bay often gets gusts of more than 105km/h and we do not see such events.
This goes to show that there are other factors that come into play when considering wind.
The Venturi effect did not play any role in the Strand with houses roofs being “blown off”, with winds far weaker than on the Du Toitskloof Pass.
Factors such as wear and tear as well as workmanship come into play here.
That, however, is a controversial subject and will be dealt with in a separate article at a later stage.
Combined dam levels:
Algoa System: 69.03%, down from 69.33% last week
This week in history:
1953: A tornado (probably a land spout) reported in East London.
Weather safety tips:
When you experience strong winds, reduce your speed and hold on to the steering wheel tightly.
Now on Facebook, Instagram, and Twitter: WEATHER GURU
E-mail garthsampson59@gmail.com with feedback or requests
HeraldLIVE
WEATHER GURU | How strong was that scary wind?
Image: SUPPLIED
Don’t confuse Ace Ventura with the Venturi effect.
I am lucky to be doing a presentation in Paarl next week and not this past weekend.
What a scary wind that was.
Most have seen video clips on social media of a trailer and caravan being blown off the bridge just past the Huguenot tunnel.
I think we all ask ourselves how strong that wind was that it could cause so much destruction.
I can even hear many “experts” shouting that the wind never gets that strong in our neck of the woods.
If I told you it was not so much the wind speed which caused the toppling of the vehicles, but more the topography and wind direction, you would be a little confused.
To explain it in the simplest of terms, imagine a parcel of air moving towards our country’s coastline.
Its speed is governed by the pressure gradient and the smooth surface of the sea does not slow it down.
When it hits the coast and moves inland, it slows due to the roughness of the land (buildings, trees and hills).
The wind in question blew from the southeast, travelling from the Agulhas coast area, where it gusted at 80km, to the Cape Fold Mountains, stretching from Swellendam to Worcester, where gusts had dropped to 50km/h.
The wind then veered to a more easterly direction along the mountainside, eventually forming a massive bottleneck in the Worcester “basin”, east of the Du Toitskloof Pass.
The Venturi effect then produced its magic and accelerated that wind to an astronomical speed capable of blowing over what looks like a 10-tonne truck’s enclosed trailer and a caravan.
By the time it hit Paarl it was less constrained and spread out over the area, by then having dropped from an unknown speed to 105km/h.
To aggravate matters, as you exit the tunnel on the Paarl side of the pass, the road gently curves to the left and then immediately into a sharper right-hand curve atop the bridge where the vehicles were blown off.
In so doing, it turns vehicles side-on into the wind, making them vulnerable to toppling, especially those with some height.
The original builders of the Du Toitskloof Pass traversed the tributary of the Krom River higher up, closer to the mountain, using stormwater pipes under the road, opposed to the Huguenot tunnel builders who traversed the river with a relatively high bridge.
Incidentally, our Bay often gets gusts of more than 105km/h and we do not see such events.
This goes to show that there are other factors that come into play when considering wind.
The Venturi effect did not play any role in the Strand with houses roofs being “blown off”, with winds far weaker than on the Du Toitskloof Pass.
Factors such as wear and tear as well as workmanship come into play here.
That, however, is a controversial subject and will be dealt with in a separate article at a later stage.
Combined dam levels:
Algoa System: 69.03%, down from 69.33% last week
This week in history:
1953: A tornado (probably a land spout) reported in East London.
Weather safety tips:
When you experience strong winds, reduce your speed and hold on to the steering wheel tightly.
Now on Facebook, Instagram, and Twitter: WEATHER GURU
E-mail garthsampson59@gmail.com with feedback or requests
HeraldLIVE
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