IELTS Academic Reading Practice Test 28: Passage 1

In some of the driest places on Earth, a great deal of water drifts past in the air. Along the coasts of northern Chile and Peru, and of Namibia in south-west Africa, cold ocean currents chill the air above them, and the moisture it contains condenses into banks of fog that are carried inland by the wind. Very little rain falls in these regions, sometimes almost none for years at a time, yet the fog that rolls in from the sea can be thick and frequent. Plants and animals have long exploited this hidden resource, and increasingly, so have people.

Perhaps the best-known example is a beetle that lives in the Namib Desert. On foggy mornings it climbs to the crest of a sand dune and stands facing the wind with its head lowered and its back raised, so that droplets of fog collect on its body and trickle down towards its mouth. Some plants obtain water in a comparable way. The giant coast redwoods of California absorb moisture from summer fog, both through their leaves and from the drops that fall onto the ground beneath them, and this helps them to survive the long dry season. In Chile and Peru, isolated patches of vegetation, sometimes described as 'fog oases', exist only where fog regularly reaches the hillsides.

The devices that people use to capture fog are remarkably simple. A typical fog collector consists of a rectangular panel of fine plastic mesh, usually made of polypropylene, stretched between two posts and set up at right angles to the prevailing wind. As fog blows through the mesh, some of the tiny droplets strike the fibres and cling to them. These merge to form larger drops, which eventually become heavy enough to run down into a gutter along the bottom of the panel. From there the water flows through a pipe to a storage tank, from which it can be distributed to homes or used to water crops. No energy is needed apart from the wind that carries the fog.

Choosing the right site is critical. The best locations are usually ridges or slopes facing the sea, several hundred metres above sea level, where the fog is dense, the wind blows steadily and there are no hills or tall trees between the collectors and the coast. Before building a large installation, organisers normally put up small standard collectors, each with one square metre of mesh, and measure how much water they catch over a period of months, ideally a full year. Yields vary greatly from place to place and from season to season: at a good site, each square metre of mesh may collect several litres of water a day on average, and on exceptional days far more.

One of the first large projects began supplying water to the fishing village of Chungungo in northern Chile in 1992. Around a hundred large collectors were eventually built on a mountain ridge above the village, and for several years they provided much of the community's water, which had previously been delivered by lorry. The project attracted international attention, but it did not last. Around 2000, local officials stopped maintaining the nets, preferring to seek a conventional water supply, and within a few years none of the collectors was working. The lesson for later organisers was that technical success is not enough: a fog-harvesting scheme needs the long-term support of the community it serves.

A more recent project in the Anti-Atlas mountains of south-west Morocco was designed with such lessons in mind. Run by a Moroccan non-governmental organisation, it pipes water directly to homes in more than a dozen small villages. Women, who traditionally spent several hours a day fetching water, were involved from the start and have been given an important role in monitoring the system, using mobile phones to report problems by text message. The project also uses a newer type of collector, designed to withstand strong winds, which captures considerably more water than the earlier nets.

Fog harvesting has obvious limitations. It is possible only where fog is frequent and winds are reliable, and the quantity of water it provides is usually modest: enough for drinking, cooking and small gardens rather than for large-scale agriculture or industry. Collectors can be damaged by storms, and the mesh must be repaired or replaced every few years. Researchers are therefore trying to make collectors more efficient. Conventional meshes capture only a small proportion of the water in the fog that reaches them, partly because much of the air flows around the panel rather than through it, carrying the droplets with it. Experiments with finer fibres, different weaves and coatings that help water to drain away quickly have produced meshes that capture considerably more.

Fog harvesting will never solve the world's water shortages. In the right places, however, it offers a cheap source of clean water that needs no fuel and does little harm to the environment, turning the clouds that drift in from the ocean into something that communities can rely on.