Wind climatology of the South Western Cape - by Ralph Hamann

Windsurfers rely on two quite selfish and temperamental friends - wind and waves. However, both wind and waves succumb to routine - sometimes more, sometimes less - which, if known, will enable pleasurable sailing experiences.

A present from the heavens

The movement of the air in the atmosphere is very complicated. Meteorologists try to explain this movement by identifying areas of similar pressure: high pressure systems, “highs”, and low pressure systems, “lows”. On a synoptic weather map these are shown by isobars, or lines connecting points of equal pressure. The air will want to move from high to low pressure across the isobars. But because of the rotation of the Earth, the air is deflected to the left in the southern hemisphere and to the right in the northern hemisphere in such a way that wind blows parallel to the isobars. (This is the mysterious and very confusing “coriolis force” - it does not determine the flow of water out of the bath sink). Thus, in the southern hemisphere wind blows clockwise around a low and anti-clockwise around a high.

For the South-Western Cape there are two weather systems which are important in producing our wind : the South Atlantic High and the Westerly Low (or “cold front”). The South Atlantic High is an almost permanent and stationary feature over the Atlantic Ocean to the west of the sub-continent. It is responsible for the strong southerly wind in the South-Western Cape - the “South-Easter” - which is similar in origin to the North-East Trade Winds of Hawaii or the Canaries.

The Westerly Low on the other hand moves from west to east passing southern Africa with a frequency of once every 2 to 8 days, and is often associated with a cold front and a well-defined cloud band which can be seen clearly on satellite images. Cold fronts have a strong effect on Cape weather in winter, bringing gale-force North-Westers and torrential rain. In summer, they normally pass to the south of the continent, but they still interfere with the weather.

The 6-day pattern

So what about the routine? There isn’t really one - the atmosphere cannot be relied upon! There merely exists the tendency for things to happen in a certain order. This order is shown by a 6-day pattern which is obtained by averaging the weather cycles associated with the passage of Westerly Lows. It is not a regular pattern - only an average - and can be drawn schematically like this :

DAY 1 - The weather is dominated by the Highs over the Atlantic and Indian Oceans. There are blue skies and southerly winds which strengthen in the afternoon. This is because the land heats up more than the ocean during the day, causing the air to rise and the pressure difference to increase.

DAY 2 - A Westerly Low approaches. This can sometimes be seen in the presence of high-level feather clouds or uniformly spaced bands of clouds high in the atmosphere. This causes strong storms south-west of Cape Town, with winds of 50+ knots, which whip up the sea, bringing big south-westerly swell to the Cape.

The Low reaches the continent on DAY 3. In winter the cold front hits the West Coast with a strong north-westerly wind and rain. In summer the fronts normally pass to the south of the continent and their affect is to interrupt the southerly wind with a westerly or south-westerly one. This affect will be felt less the further you go up the West Coast.

DAY 4 - As the front moves to the East the wind switches to a south-westerly and the sky clears up. Later on, the passing low sucks the South Atlantic High in behind it, causing a southerly wind that is strong and cold. The first two days after the passage of a cold front are often the best for wavesailing.

DAY 5 - Full blown South-Easter from Cape Point to Angola!

DAY 6 - The High has been pulled along the south of the continent. This often means that it is a hot and windless day in Cape Town but it still blows at Cape Point. The High then thins out and the weather map goes back to that of day 1.

So now we know when wind and waves like to hit town. The remaining question is where best to meet them. The South-Western Cape offers a small-scale wind-climatology that will perplex any sailor or scientist. How often have we had a dismal rainy day in Cape Town, but blue skies and 20 knot winds in Langebaan. Or even more common, no wind at Blouberg but almost too much at Cape Point! So don’t get stuck in routine!

Comments

By Anonymous @ Tuesday, January 17, 2006 2:43 AM

Very useful.
And well timed.
I arrive in cape Town this Wednesday!

basher

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