What if snow fell in the desert?

When you think of a desert, you might think of hot sand.

But what if snow fell on the sand? Could that ever happen?

Most people picture deserts as blazing hot seas of sand. So snow in a desert sounds impossible, like a snowman on a beach in summer. But what if snow fell in the desert?

Here's the surprise: it has happened, even in the Sahara. This lab follows three paths: real snow on the Sahara, why deserts can be so cold at night, and what cold and snow mean for desert plants. Each card says whether it's a fact or an idea.

The premise sounds contradictory only if deserts are equated with heat. USGS defines them by dryness: they 'may be hot, they may be cold', but are always dry.

This lab examines documented Saharan snowfalls, the physics of cold desert nights and the role of freezing in limiting desert plants, separating observations from ideas about what more frequent snow might do.

Deserts are very dry. They get very little rain or snow.

Some deserts are hot. Some are cold. One very cold kind is called a polar desert.

A desert is a place that gets very little rain or snow. The U.S. Geological Survey says deserts can be hot or cold, sandy or rocky, but they are always dry. Arid lands get less than 250 millimeters of rain a year.

Some deserts are cold for much of the year. Polar deserts, like the Dry Valleys of Antarctica, cover nearly 5 million square kilometers. And midlatitude deserts, like the Sonoran Desert in North America, have a wide range of temperatures through the year.

USGS: arid lands receive under 250 mm of precipitation a year; extremely arid lands go at least 12 consecutive months without rain. Polar deserts, with under 250 mm a year and a warmest-month mean below 10 °C, cover nearly 5 million km², mostly bedrock or gravel plains, where freeze–thaw cycles pattern the ground.

Midlatitude deserts between 30° and 50° N and S lie in interior basins far from oceans and have a wide annual temperature range. Cold is a normal part of many deserts.

Snow has fallen on the Sahara Desert! In 2018, people slid down snowy sand dunes.

By the next day, most of the snow had melted away.

Snow has really fallen on the Sahara. On December 19, 2016, snow fell on the Algerian town of Aïn Séfra, sometimes called the gateway to the desert. NASA notes that the town's last recorded snowfall before that was in February 1979.

Then on January 8, 2018, it snowed there again. News reports said 10 to 30 centimeters piled up on some higher desert areas, and people slid down snowy sand dunes. Warmer temperatures melted much of it within a day.

NASA Earth Observatory documented snow at Aïn Séfra, a town of about 35,000 people (2016) between the Atlas Mountains and the Sahara's northern edge, on December 19, 2016, the first recorded there since February 1979; MODIS images from earlier Decembers showed snow is unusual in that part of the Saharan Atlas.

On January 8, 2018, Landsat 8 imaged a second event: reports gave 10–30 cm on some elevations of 1,000 m or more, much of it melting within a day. Both events were near the Atlas Mountains.

At night, the ground lets its heat float up into the sky.

When there are no clouds, the heat escapes fast. Clear desert nights can get chilly!

At night, the ground loses heat to space. The National Weather Service explains that this cooling is stronger when the sky is clear. Many deserts have clear skies, so after sunset their warmth can slip away quickly.

Height matters too. Air gets cooler as you go up a mountain: in Death Valley, the National Park Service says temperatures drop about 2 to 3 °C for every 300 meters you climb. The 2018 Saharan snow piled up on higher desert ground, 1,000 meters or more above sea level.

Radiational cooling, the net loss of heat from the surface to space at night, is more pronounced under a clear sky (NWS). Deserts are often clear, favoring large night-time drops, though not always: Death Valley's deep basin traps heat, and summer lows may stay at 30–35 °C.

Elevation also cools: NPS gives 2–3 °C per 300 m in Death Valley. The January 2018 snow at Aïn Séfra accumulated on ground about 1,000 m or more above sea level.

The tall saguaro cactus does not like long, hard freezes.

That's why saguaros grow only in the warmer parts of their desert.

Some desert plants are tough against cold, and some are not. The giant saguaro cactus of Arizona is one that cold can harm. National Park Service scientists studying saguaros found that freezing weather was the most damaging weather factor for them at the cold edge of where they grow.

Some desert animals are ready for cold, too. Bactrian camels in the Gobi Desert grow a shaggy coat in winter to protect them from freezing cold, and shed it in the hot summer.

NPS Scientific Monograph No. 8, Ecology of the Saguaro, describes freezing weather as the most periodically damaging and ultimately limiting density-independent climatic factor for the saguaro at Saguaro National Monument and elsewhere at the edge of its Arizona range, where extremes rather than averages control distribution.

San Diego Zoo Wildlife Alliance notes that Bactrian camels, native to the Gobi Desert and Mongolian steppes, grow a shaggy winter coat against freezing cold and shed it in the hot season.