On the Cape Point Peninsula we have two prevailing weather patterns- the south easter and the south wester. Usually these two winds can be catagorized by the south wester being the prevailing wind in Cape Points winter season and the south easter being the norm for summer.
South wester is a strong indicator that bad weather is on the way, it normally blows before and after cold fronts come in off the Atlantic. It is not always a rain bearing wind, though more times than not it is.
South easters in the summer are locally known as the Cape Doctor because they keep the temperatures in the area pleasant rather than oppressive. Cape Point Peninsula residents have a love hate relationship with the Cape Doctor. When it blows we curse the strong winds, but when it stops (especially in the middle of summer) we lament its absence as we swelter in high temperatures.
Though this is a predictable weather pattern the south eastern isn’t always so benign. This weekend it is showing its other side. When conditions are just right (or wrong, depending on your point of view) they conspire to produce something called a ‘black southeaster’. The wind blows unusually strongly and the usual dry, sunny weather is replaced with very high levels of rain fall. Its like a winter Atlantic storm in reverse.
The Cape Doctor is caused because of a pressure difference between a low pressure system that spends th summer planted over the highveld and a high pressure that slips down the atlantic coast from Nambia and positions itself in the Atlantic somewhere to the west of Cape Point. Winters south westerly winds slip further south towards the pole.
Normally the winds are dry, though usually a little cool. When conditions are just right a surface low pressure system squeezes into the area between the ever present high and low pressure system. When this happens the Cape Point Peninsula is in for a wild ride. Winds up to 100mph have been recorded during Black South Easters and the phenomina is responsible for severe flooding in many low lying areas. Trucks are often blown over and wind tunnel effects between buildings in Cape Town CBD have caused people to swept off their feet as well.
Most at risk are the low lying communities of the Cape Flats, usually the poorest communities living in make-shift shacks, often in areas that are not deemed suitable for habitation (because of the risk of flooding). The effect was also responsible for the Laingsburg floods of 1981 and 1994, when deluges of rain caused flash floods of the Buffels River (click this link to learn more about the Laingsburg floods).
Last night was typical of what to expect during a Black South Easter. High, squalling wind speeds coupled with driving, torrential rain. I ran out to the garage and back and was wet through in seconds. As I look out of the window now there is a steady drizzle, though the weather service says we can expect some more later. The good news is that summer is just around the corner and we can go back to sunshine from dawn to dusk again on the Cape Point Peninsula.