Academic Reading — Test 149

Reading passage

Along the south-western edge of Africa, where the Atlantic Ocean meets the arid shores of Namibia and the northern Cape, lies one of the foggiest coastlines on Earth. Here the cold Benguela Current sweeps northward from the polar latitudes of the Southern Ocean, carrying chilled water past a desert that receives almost no rain. The meeting of cold sea and warm land produces a thick, persistent sea fog that rolls inland on most mornings. For the plants and animals of the Namib Desert, this fog is not a curiosity but a lifeline, supplying moisture that the sky otherwise withholds. The Benguela Current is what oceanographers call an eastern boundary current. Such currents flow along the western coasts of continents and tend to be cool, because they draw water from higher, colder latitudes. The Benguela is made colder still by a process known as upwelling. Persistent winds blow along the coast from the south, and the rotation of the Earth deflects the surface water away from the shore. To replace the displaced water, cold water rises from the deep ocean. This upwelled water is rich in nutrients, which is why the Benguela supports vast shoals of fish. It is also several degrees colder than the surface water found at similar latitudes elsewhere, and this coldness is the engine that drives fog formation. Fog is essentially a cloud that forms at ground or sea level. It appears when air is cooled to its dew point, the temperature at which the air can no longer hold all of its water vapour as an invisible gas. The excess vapour then condenses into countless tiny droplets suspended in the air. Along the Benguela coast, the relevant process is called advection fog. Warm, moist air moving in from over the warmer parts of the Atlantic passes across the cold band of upwelled water near the shore. The lowest layer of this air is chilled from below until it reaches its dew point, and a sheet of fog forms over the sea. Unlike radiation fog, which develops on calm, clear nights as the land loses heat, advection fog requires the horizontal movement of air and can persist even when a light wind is blowing. Once formed, the fog is carried onto the land by the daily rhythm of coastal breezes. During the day the desert heats rapidly, and the warm air above it rises, drawing the cooler, fog-laden marine air inland. The fog typically arrives in the small hours of the morning and lingers until the sun climbs high enough to warm and disperse it, usually before midday. Its reach is limited: the fog rarely penetrates more than fifty to a hundred kilometres from the coast before evaporating in the heat of the interior. Within that belt, however, it is remarkably reliable, occurring on more days of the year than rain falls in a decade. The biological importance of this fog can hardly be overstated. The Namib is among the oldest and driest deserts in the world, yet it teems with specialised life. Certain beetles climb to the crests of dunes at dawn and tilt their bodies into the wind so that fog droplets collect on their backs and trickle down to their mouths. Lichens, which cannot draw water from the parched soil, absorb moisture directly from the damp air. Several desert plants have shallow, wide-spreading roots that gather the thin film of water deposited on the sand each morning. In a landscape where measurable rainfall may not occur for years at a time, fog provides a steady, if modest, supply of water that countless organisms have evolved to harvest. Human communities have learned from these natural strategies. In parts of the region, large mesh panels known as fog nets have been erected on hillsides to intercept the passing mist. As the fog drifts through the fine netting, droplets gather on the threads, run down into gutters and are channelled into storage tanks. The water collected in this way is clean and requires little treatment, and the technique uses no fuel or electricity. Although the volumes harvested are small compared with the demands of a city, fog nets can supply enough water for a village or a remote school, offering a simple and sustainable resource in places where conventional supplies are scarce. The same cold current that makes the coast so forbidding thus becomes, indirectly, a source of life.