Martin Moore learns about the winds of cool Durbanville

One’s never too old to learn. How often does one not remind oneself of that homily when you discover what you have always taken for granted is only partly true? I have always believed that, apart from location, the major reason why Durbanville is one of the two coolest regions in the Winelands is because of the cold south-easter that blows off False Bay in summer, whipping across the Cape Flats and swooping over our hills to cool down the vines (nicely said, Martin). And anyone that has stood with me behind the cellar when the grapes are being off-loaded would have agreed that that is a cold wind indeed.

So we have always been looking towards False Bay in the south-east when the vineyards craved relief from the summer heat, ignoring False Bay right on our doorstep to the west. Of course, other winds also blow in summer, but for me they were of less consequence. How wrong I was, and how much more complex is nature, as I discovered from the August edition of Wynboer in which appeared the results of a seven-year study on mean February temperature in our wine-growing areas to help determine what cultivars should be planted where.

The researchers found the wind that blew most frequently in our hills was not the south-easter. No, in 27% of the cases it was a westerly wind coming off the cold Atlantic compared to 22,5% of the time when the wind came from False Bay in the south-east. And the combination of the two results in a strong land-sea breeze circulation over the area, a phenomenon in which Table Bay plays a much greater role than we previously thought.

That's not the end of the story. Our hills are 12 km from Table Bay and 27 km from False Bay, jutting out like a ship' prow over the low-lying Cape Flats and fully exposed to the winds from both directions. Sea breezes can penetrate up to 100 km inland, but on the way they also get warmer, at a rate of 0,6°C every 10 km. Which means when the westerly winds get to us, the temperature in theory is about 0,7°C higher and, in the case of the south-easter, about 1,5°C. However, height above sea level also plays a role, cooling down the wind at a rate of 0,5°C for every 100 metres gained in height. Our highest vineyards are at about 400 metres, so that by the time the wind reaches them, it has, in fact, cooled down to below its original temperature.