It Did Not Rain in This Town for Fourteen Years

Between October 1903 and January 1918 no measurable rain fell in Arica in northern Chile. 172 months in a row. It is the longest documented dry spell of an inhabited town.

Arica sits at the edge of the Atacama, which stretches for some 1,000 kilometres along the Pacific coast of South America. In the hyper-arid core of the desert there are areas with no recorded rainfall at all since measurements began. The weather station at Quillagua averaged half a millimetre of rain a year. In Germany, depending on the region, 500 to 1,000 millimetres fall.

Three mechanisms working together

Figures like that do not come from a region lying far from the sea. The Atacama is right on the coast, and from many places you can see the Pacific. It is dried out by three processes that interlock.

First, the rain shadow. The Andes rise to the east. Moist air from the Amazon basin is forced upwards against them, cools and drops its rain on the eastern flank. What arrives on the western side is already dry.

Second, the cold Humboldt Current. It carries cold water northwards along the coast. Little evaporates over cold water, and the air above it cools instead of rising. Fog forms and drifts up to the slopes, but hardly a rain cloud does.

Third, the subtropical high-pressure belt. At these latitudes air sinks over wide areas. Sinking air warms up and can take on more moisture instead of releasing it. Cloud formation is systematically suppressed.

Each of these processes on its own would keep the region dry. Together they seal the Atacama off from any air that could bring rain: no moisture crosses the Andes from the east, none rises over the cold water in the west, and from above the sinking air presses down whatever does form.

The desert lies by the sea. It is dry all the same, or precisely because of it.

— TerraTally · Climate desk

None of this makes the Atacama empty. It holds towns and ports, along with some of the largest copper mines in the world. Their water comes from rivers running down out of the Andes, from groundwater and from desalination plants on the coast that treat sea water. Water there is no side issue of planning but the condition for any work or settlement in the desert at all.

And there is a resource that would be none anywhere else: fog. In some places nets are strung across a slope. The fog drifts through, droplets settle on the mesh, run together and drip into containers. Humidity that never adds up to rain becomes drinking water this way.

That is what makes the Atacama more interesting than its record figures suggest. It is not a hostile extreme, but a place where people have learned to get by on almost nothing. The methods they developed are only now beginning to be needed elsewhere.