Despite our initial take on what a desert looks like, a sprawling searing sandscape as far as the eye can see, the definition is a bit simpler than that. Deserts are characteristically dry regions receiving less than 250mm of precipitation over the course of a year. A common term to describe deserts is "arid" which applies to both the atmosphere and, importantly, the soil of the region. Another way people like to consider a region a desert is "an area unfit to sustain a human population," but that is a highly human-oriented view of the planet.
You might notice that there is no sand qualification for the desert, and this is true. While sands have a habit of forming in deserts, there are plenty of desert regions where any sand gathering doesn't occur, leaving only the exposed plains of stone and bedrock.
To be a desert, a region can't have more than 250mm (10in) of precipitation in a given year
The Sahara Desert in Africa is the Earth's largest non-polar desert coming in at a whopping 9.1 million square kilometers (3.5 million square miles)!
The Earth's "trade winds" are winds that flow from the tropics of cancer and capricorn to the equator. As these winds move, they get warmer, causing cloud cover to disperse. This has a twofold effect of reducing our planet's water transport and creating a hotter, dryer atmosphere. A perfect mix for the arid conditions needed for a desert! Africa's Sahara desert is not only the largest non-polar desert, but also an example of a trade winds desert.
Monsoon and Midlatitude
Another vector for desert creation is being far enough inland that precipitation is simply an uncommon occurrence. Reflecting on the water cycle, we can recall that rainfall originates in our large bodies of water and is effectively blown inland by our planetary winds. Midlatitude deserts like the Sonoran in North America are just too far inland for precipitation to be carried over the desert. Whereas monsoon deserts like the Thar in Asia are just beyond the circulating winds of the monsoon area, resulting in little rainfall reaching the desert.
And, back to what we mentioned earlier, deserts don't have to be hot to be deserts. The polar regions of the Earth have yearly high temperatures of 10 degrees Celsius and meet the precipitation requirements for deserts coming in at under 250mm per year. Additionally, cold deserts are the largest deserts on Earth with the Antarctic Desert ringing in at 14,200,000 square kilometers. That's the size of the United States 1.5 times over!
Even though deserts are defined as arid places making it difficult for plants to take hold, our fellow species on Earth are a proven resilient bunch. While not as biodiverse as a rainforest, the desert biome sports an assortment of plants and animals finely tuned to living in a hostile environment.
The Atacama Desert in South America is the driest desert on Earth, only receiving 1mm of rainfall ever 5-20 years!
Despite old beliefs, camels store fat in their humps, not water
There are also a number of birds that inhabit the desert arena ranging from woodpeckers that nest in cacti to the popularized roadrunner which prefers its feet to its wings. And despite not being the first thing to think of when it comes to deserts, a fair number of mammals have adapted to the environment. From small burrowers such as rats to foxes and jackals all the way to the mighty lion, mammals prove that there can be a space for them in even the toughest regions of the world.
We'd also be remiss not to mention the camel, a particularly min-maxed mammal when it comes to desert living. To prevent itself from overheating, the camel stores all of its fat in its characteristic humps. This also acts as a nutrient store for the camel when it is unable to eat or drink for long durations of time. These features of the camel have earned it the title of the "ship of the desert!"
Definitely a newer consideration is the use of deserts for solar energy capture. Over the course of this page, we've seen that one of the core reasons deserts are arid is due to lack of precipitation in large part because clouds just don't reach deserts. Without cloud cover, these areas can take full advantage of the sun's incoming energy like no other region on the planet.
Another particularly interesting benefit is the inherent resilience of species that have grown to survive in these regions. Throughout history, even into today, humans get their inspiration from studying the world around us, and the species found in deserts are some of the most specialized creatures on the planet. There is likely a ton we can learn by focusing research efforts on deserts and the species that inhabit them.
Of course, this is all a balancing act. Deserts that have naturally formed over geological time take hold due to the reach of the Earth's functions and systems. Just because they're here doesn't mean they're the most conducive areas for a majority of the life on our planet. And wouldn't you know that humanity has a way of teetering the formation of deserts as well...
Desertification is the process by which overcultivation of land leads to arid soils and desert expansion.