A worker was looking after Rob and Carol Ball’s house, near Pelenechi Manor, while they were on holiday and heard a loud crash on that particular Friday evening. She got a fright but decided to investigate and found two glass doors lying shattered on the ground below the patio.
Ball estimates the damage to be around R5 000 and says he found the occurrence to be quite unusual.
Readers were invited to hazard a guess as to how it happened and a reader named LeGrange sent in an email detailing similar occurences and explaining the phenomenon.
“We grew up in the Free State with its August winds and, since we were little, were taught to keep a window open on the wind’s offside. Even if dust blows in, it just gets swept out again.
A new teacher moved to our area. He complained about the dust and how he keeps everything closed and we cautioned him to keep a window open.
With the next gust of wind, he tells us that he didn’t heed the warning and closed up all the windows as usual. Halfway through the storm he though about the warning and went to open the bathroom’s window. The wind that blew out of his house, as the air pressure inside the house tried to stabilise, tore the window from his hand and made it hang in the air for a few moments before it fell back down.
The theory behind the phenomenon is that every house has a certain level of air pressure. When a storm (with strong winds) starts raging, people usually close all the windos and doors and that pressure is kept inside the house. The wind lowers the air pressure and, especially on the offside of the house, could cause a vacuum. This would explain the suction effect on the patio doors – the air pressure inside the house was much higher than the outside and it tried to stabilise, blowing off the doors.
Often, the form of the roof and the angle at which the wind blows against it, is also enough to cause a vacuum. If all the doors and windows are closed and the pressure inside the house builds up enough, the roof is lifted and blown away.”*
*The email was translated from Afrikaans and care was taken to preserve the original intent and meaning.
