Creating realistic terrain involves more than simply sculpting out a feature and plonking it on the landscape. To truly create a believable landscape, the process of natural desert formation has to be understood, so that deserts can be created in believable locations.

Defining a desert

A desert is defined as places where almost nothing lives. They are known as deserts, because they are deserted.

Deserts are driven by the weather. The flow of water on the landscape around the desert is what defines the size and shape of any desert. This means that surrounding landscape has to be taken into consideration, as does the weather.

Hot Deserts

Hot deserts exist in warm locations, usually at or near the tropics. They are covered in sand, rock, or small stones. The land is often good quality, except it is bereft of water, which is what makes the hot desert so dangerous. Hot deserts do not have to be rolling sand dunes, and can be flat and rocky just as easily. These deserts are typically extremely hot during the day, offering little or no shade, yet plummeting quickly in temperature at night. These conditions: rapid heating and cooling, are extreme enough to break rocks.

Cold Deserts

Cold deserts are actually covered in water. The problem is, this water is all frozen, way below zero centigrade - life cannot exist in frozen water. Cold deserts are typically where the air is very cold, at the caps of a world, or the tops of high mountain ranges.

Ash Deserts

Sometimes deserts form around active volcanoes. Ash not only fills the lungs of most animals, choking them, and blocks sunlight, but the fine ash particles will bond with water in the air, dropping it as acidic rain. If an eruption is ongoing, a small desert will form in the ash shadow, a desert that is normal temperature, covered in extremely fertile ash, but completely bereft of water.

Poison Deserts

A fourth type of desert is the poison desert. These crop up when the landscape is filled with a substance poisonous to life. No matter how much water there is, if it is mixed with poison, nothing will live there, and a desert will form. The dead sea is an example of such, where a sea filled with water has such a high salt content, that absolutely nothing can live there. Other poison deserts include salt flats, and even ocean deserts, where effluent has mixed with the seawater, killing off everything that grows along the current's path.

The formation of Deserts

Deserts form when there is either not enough water for anything to grow, or the water that is available is unusable due to temperature, acidity, or other toxins. Whatever the reason, the main contributing factors are water and wind.

Rain Shadow Desert

Water does not sit still. If a world has weather, it has moving water, continually circulating. Water evaporates from any standing body, and flows through the air into greater and greater clouds, rising and rising until it meets colder air, at which point it rapidly condenses and falls as rain. If the clouds rise too high, the water freezes and snow forms. The height of the land forces clouds higher, which is why it snows over mountains.

If the prevailing wind is blowing east, then water collected from bodies of water to the west will evaporate into clouds, and rise over land heading east. Some will fall back as rain, some will survive to reach the mountains, and most will deposit on the western slopes of those mountains. The wind will flow over the mountains, but the last of the moisture will have frozen and fallen as snow or sleet. It will hardly ever rain on the eastern slopes, or the lands to the east of the mountains - thus, a rain shadow desert will form.

Distant Sea Desert

Most of the larger deserts hot are a long distance from any major body of water such as a lake or sea. Any moisture picked up on the wind, is rained back to earth long before it reaches the desert land, so the desert stays hot and dry.

Near Sea Desert

Paradoxically, a desert can be found near a large body of water. This occurs when the prevailing wind direction moves across the desert first, then over the water. Any water evaporating into the air from the sea, meets a warm land breeze and condenses into mist. No water is left for rain, so a desert stretches back.

Desert Erosion

As you can see, the common feature in desert formation is always water and wind. Not enough water, or water blown to high so it freezes, or pollutants blown into the path of the water, or water raining on stretches of toxic material wikl create a desert. The lesson to remember is the surrounding landscape and the weather patterns your world will have, determine the formation of deserts, not the other way round.

The weather itself transforms the appearance of hot deserts. When rain does fall, it falls in short, fierce storms which wash away all top layers of soil and sand, exposing the bare rock beneath. Later wind storms whip up the remaining sand, blasting it across the rock, eroding it at speed. Very little is more abrasive than fast-moving sand.

Canyons

Canyons are formed in some deserts. They occur where hills have had their edges and soft soil eroded away by the blowing sand, which works in from the sides far more than the top; eventually leaving misshapen pillars of rock where the core of the hill once was. Softer rock wears away faster, sometimes leaving holes or canyons.