Snow Falls in the Sahara

The Sahara is the classic example of somewhere hot. National Geographic Education describes it as the largest hot desert on Earth, where summer temperatures can climb above 50 degrees Celsius. So the photographs cause real confusion: orange sand dunes with white snow lying across them, like icing on a very large cake. Many people who see them for the first time assume they must be fake. They are not. Snow really has fallen on the edge of the Sahara, and satellites have recorded it from space.

The place most often in the news is Aïn Séfra, a town in northern Algeria. NASA’s Earth Observatory describes it as a town of about 35,000 people, sometimes called the gateway to the desert, that sits between the Sahara and the Atlas Mountains. On 19 December 2016, snow fell there, and NASA reported that this was the town’s first snowfall since February 1979. The Landsat 7 satellite captured an image of white snow lying over the caramel-coloured landscape nearby.

It did not stay long. NASA compared that picture with a second image taken by the Landsat 8 satellite about eight days later, and by then almost all of the snow had disappeared, except on the highest ground. The desert had taken its colour back. For anyone who missed it, the event might easily have seemed like a one-off, a strange afternoon that would not be repeated for decades.

Then it happened again. According to NASA’s Earth Observatory, on 8 January 2018, anywhere from 10 to 30 centimetres of snow built up on some of the higher desert areas near Aïn Séfra, at elevations of 1,000 metres or more above sea level. It was the second time in three years that snow had settled there. The Operational Land Imager on Landsat 8 captured the scene from orbit, and warming temperatures melted much of the snow within a day.

The key to understanding it is that a desert is defined by dryness, not by heat. National Geographic Education explains that a desert is simply a place with very little precipitation, usually around 25 centimetres a year or less. By that definition, the largest desert on Earth is actually Antarctica, the coldest desert of all. A desert can be baking hot, freezing cold or both at different times, and snow is simply rare in the Sahara rather than impossible, especially on higher ground at its edges.

So the next time a picture of snow-covered dunes appears on your screen, there is no need to assume a trick. The Sahara is enormous, and parts of its edge sit high enough, near enough to mountains, for snow to fall on a cold winter day. It rarely lasts, and it will not turn the desert into a ski resort. But for a few hours at a time, one of the hottest places on the planet has worn a thin white coat, and NASA’s satellites were watching when it did.