As a weak coldfront approaches the Cape, a low-pressure system along the coast draws hot air off the interior, resulting in a ‘berg ’ wind, with temperatures at Cape Town surging to over 30°C, before dropping by more than 15° as the front passes by. (Satellite images courtesy of the South African Weather Services)
It is usual to have delicate high cirrus clouds – often as lacy ‘mackerel skies’ – preceding the fronts. And, before this, the passage of hot, dry ‘berg’ winds rushing off the interior plateau is a frequent phenomenon.
A point of enduring curiosity is why Kirstenbosch receives over 1 300 millimetres rainfall per annum, when the city centre, just 10 kilometres distant, receives only 520 millimetres. One might expect that the strong westerlies and northwesterlies of the cold fronts would drop their heavy load of rain on central Cape Town as they reach the steep wall of the Table Mountain complex, which faces the oncoming storms.
Kirstenbosch National Botanical Garden Trip Itinerary Photo Gallery
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But the ridge of mountains is too low and too narrow to stop the rain-bearing clouds, which are forced upward and, rapidly condensing, precipitate on the opposite side -precisely above Kirstenbosch. As a front approaches, clouds cascade through Constantia Nek and over the gap between Devil’s Peak and Fernwood Buttress, followed by strong winds and pouring rain.
On some occasions the winter gales wreak havoc – in September 2001 the metal roof of the recently completed Visitors’ Centre was ripped off. Fortunately the storm struck in the middle of the night, with no people on the site – had it happened a few hours later there might have been fatal consequences.
Whereas winter storms can rage over the Cape Peninsula for several days with each passing cold front, the grey, cold weather is often broken by gloriously sunny, champagne days in mid-winter. A contrasting situation arises during spells of hot summer weather. The dominance of strong southeasterly winds in summer forces moist air up and over Table Mountain. A band of stratus cloud forms the famous ‘tablecloth’ that pours over the lip of the Table, evaporating as it mixes with the warm dry air rising off the basin in which the city of Cape Town lies. Kirstenbosch is largely protected from the onslaught of the strong southeasterly summer winds – an island of calm within the storm – but there is enough of a breeze to bring relief on hot days. On the summit, the condensation of summer clouds on the surfaces of plants and rocks, when added to the winter rainfall, effectively doubles the annual deposit of moisture – a phenomenon first accurately measured in the 1890s by that pioneer of fynbos ecology, Rudolf Marloth.
Four seasons in one day Delicate high cirrus clouds announce the arrival of a cold front on an early spring day. Seen flowering are Bulbinella latifolia (yellow), Lampranthus spectabilis, Lampranthus amoenus (pink) and Drosanthemum speciosa (orange). Capetonians are given to saying that, if you don’t like the weather, wait around for a few hours.
The weather station at Cape Town International Airport weather station has recorded changes of over 20°C in just a few hours, especially as hot ‘berg’ winds from the interior arrive on cool winter mornings. This area was aptly named the ‘Cape of Storms’ by Portuguese navigator Bartholomeu Diaz some 500 years ago, and the dynamic nature of the area’s weather often produces four seasons in one day. At the popular Kirstenbosch Summer Sunset concerts, visitors in light summer dress soon find themselves searching for a fleece as the afternoon sun disappears behind Maclear’s Buttress and cold air rolls down the mountain slope. This cool, moist, microclimate provides the required conditions (as well as protection from fire) for the indigenous Afrotemperate Forests that clothe the ravines above the Garden, and permits the planting of a wide diversity of species from the higher reaches of the Cape Fold Mountains – home to the richest diversity of the Cape flora. On a foggy autumn morning, the lower garden is covered by a damp shroud while the upper garden is bathed in sunlight. Characteristic, too, of the Kirstenbosch climate is the mist and fog shrouding the Garden’s lower reaches on many autumn mornings. Early visitors climbing up Nursery Ravine are delighted to emerge from the foggy cover of the lower Garden slopes into the bright morning sunlight, with the Cape Flats still hidden below the mists.