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Thu, 11 Mar 2010

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WINTER RAINFALL

Rainfall around the Cape Peninsula and the Boland varies greatly in space. Newlands is one of the wettest suburbs in the country with an annual rainfall of well over 2000mm (80 inches) while only 30km (18mi) away within view the Cape Flats (Airport) only receives about 550mm (22 inches). Jonkershoek near Stellenbosch also receives very high rainfalls and holds the South African annual rainfall record of over 4000mm (160inches). The diagram below shows how the rainfall is distributed around the CT area.


 - RED    =    0-500mm

 - YELLOW =  501-1000mm

 - GREEN  = 1001-1500mm

 - BLUE   = 1501-2000mm

 - PURPLE = > 2000mm

So what causes these great differences in the rainfall ?

Topography, i.e. the mountains play a very significant role in the distribution of rain and also the wind direction.

Usual geographic theory tells us that when a moisture laden onshore wind encounters a mountain it will rise and moisture will condense and rain will occur on the windward side. The leeward side remains dry as the moisture is not able to rise up and over the mountain, which acts as a barrier. Hence the leeward side is usually very dry. This is typical of the George-Oudtshoorn area. This is depicted in the diagram below

However, let us take the usual scenario in Cape Town in winter when a fresh to strong north westerly wind is blowing. Since Table Mountain is not too high, the airflow is able to go up and over the mountain. Table Mountain therefore acts as a trigger mechanism, i.e. it forces air to rise up higher than it would have if it had not encountered the mountain. As it is forced up, the moisture condenses and most of the rain falls over Newlands, where the uplift is strongest.

As the airmass moves away from the mountain the upward forcing is no longer there and the rainfall also decreases proportionally as one moves away from the mountain. This is why Newlands and most areas such as Wynberg, Constantia and Claremont get so much rain in winter compared to other regions in and around Cape Town. Also, the mountain causes great wind turbulence in it's wake, hence these same areas also get some of the strongest winds in the Peninsula during really bad north westerly gales.

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