Note: I have a lot of research on this topic from my worldbuilding on A Sign in Blood, so I'm going to have to split this up into a few posts. Right now, I'm planning on two, but that could change once I really dive into it.
Part 1 | Part 2
Part of worldbuilding is knowing where to put things. After all, different types of geography don't just appear randomly and fully formed on the face of the earth (or any other planet). Usually. Everything we see around us is the natural result of the forces exerted by the sun, currents, rain, and even the Earth's rotation in space.
Understanding these to some degree helps a writer to build a world. And using geography to do this isn't just a matter of accuracy or realism. Understanding different landforms helps a writer by giving them options they may not have considered, or even heard of. When you're writing about something you've never experienced—as we all often are—it's better to go in armed with knowledge and some idea of what you could be creating, rather than later realize that your vast, sweeping desert is really a giant sandbox.
Knowing what the options are here, and how those options came to be, frees you up to get creative and gives you the tools to build something really spectacular. So, without further ado, here are some ways in which deserts are created.
Basic Desert Formation
Deserts generally form on either side of the equator because the air there is exposed to intense sunlight and it heats up. There is a lot of ocean at the equator, and under this intense heat, a lot of water evaporates. This air, both hot and moist, rises up in the atmosphere, gradually cooling as it does. Hot air can hold more moisture than cool air, so the cooling rings the moisture out of it. This precipitation falls on the land at the equator, producing the tropics. The now dry, cool air then descends again about 30 degrees away, on either side of the equator. It warms and expands, becoming even drier for being rewarmed, and this is one way in which deserts form. The land in those areas of dry, rewarmed air doesn't have the moisture to support much, if any, plant life.
There are some things to consider when building your world. This cycle happens because of the vast among of water at the equator. If your world doesn't have as much water at the equator, or has more, you'll need to adjust your world's weather accordingly. In addition, if your world has a weaker sun, for some reason doesn't get much sun at the equator, etc., you'll have to adjust for that as well.
Also, deserts, just like other types of geography, have landforms (which we'll talk more about in part 2) and characteristics that come from the ways in which they were made. And these can vastly affect and the look and feel of your desert, which can vastly affect the way you describe your desert and the things your characters are likely to encounter while in them.
Another way for a desert to form is if the area is far inland from the ocean. Most of the moisture in the atmosphere is evaporated from the oceans, and then falls on land. As the air moves in toward the center of a continent, it gets more and more dry and, by the time it reaches the farther inland destinations, it's got no more moisture to give. This is the case with both the Gobi and Takla-Makan deserts.
The "rainshadow effect" can also create a desert. In this case, a mountain range has a windward side against which moisture-bearing air flows. This air is then pushed up higher into the atmosphere by the mountains. As it goes higher, the air is cooled and the moisture condenses, forming precipitation on the windward side. However, on the leeward side (the side not exposed to moisture-bearing air currents) the already dry air descends, becomes warmer, and so drier still. This can create a lush forest on the windward side, while leaving the leeward side, and the ground for some distance beyond, a dry steppe land or desert. To get an idea of these types of deserts, think of the American Southwest.
Other types of deserts are caused by coastal cooling. Air moving over cold water becomes cold and therefore cannot hold as much moisture. As it moves over land, it rewarms, becoming drier still (especially if there is no moisture to absorb as it rewarms). In such areas, such as the Atacama and Kalahari deserts, fog or mist often occurs along the coastline (and inland to a certain extent), but rain rarely does.
While any of these forces can, by themselves, produce a desert region, they often combine and this can make a desert even more severe.
Next time we'll talk about desert heat, and the lack of it, and landforms in the desert (unless that turns into two posts).