Desert snow

A rare fall of snow has blanketed parts of Chile’s Atacama Desert, surprising visitors and residents alike.

The weekend snowfall in the desert city of San Pedro de Atacama, 1,200 kilometres northwest of Santiago, was the heaviest in 30 years.

In the coastal town of Tocopilla, melting snow combined with the desert soil to produce a mudslide which stopped just short of some houses.

Heavy machinery was brought in to clear parts of the coastal highway and to help free trapped motorists.

The snow came from a cold weather front which had spread up from the Antarctic. It brought heavy rain and snow to a swathe of South America, from Peru in the northwest to Paraguay and Brazil in the southeast.

Heavy snowfall was reported in Bolivia’s capital, La Paz and temperatures were 8 or 9 degrees below average.

The equivalent of a month’s worth of rain fell in six days in Porto Alegre, Brazil. Almost 7,000 people were forced to leave their homes after flooding in several towns in the state of Rio Grande do Sul.

The Atacama is generally regarded as the driest place on Earth. Although the average annual precipitation is around 15 millimetres, some weather stations have never recorded any rainfall. It is thought that no significant precipitation was recorded in the desert between 1570 and 1971.

The desert’s aridity can be explained by a combination of its location and climatology.

The Atacama lies to the lee of the mighty Andes mountains, which effectively cut off any supply of moisture from the Pacific Ocean.

The mountains also block any showers which might penetrate the region from the Amazon basin to the north.

The cold waters of the Humboldt current along the coast, and the South Pacific high pressure zone, combine to produce a zone of subsiding, dry air.

So it takes an exceptionally vigorous weather system to break through those barriers. One such event occurred in July 2011 when an Antarctic cold front brought as much as 80 centimetres of snow across the region.

No further snow is predicted for the Atacama. San Pedro de Atacama can expect daytime temperatures into the low twenties Celsius, but nighttime temperatures will still fall below freezing.