Deserts take up a third of Earth’s land surface but can be larger or smaller on the world we’ve created. They play a role in regulating Earth’s temperature, so deviating from this could render our world less habitable, but our audience isn’t likely to know that.
While lack of rain causes some deserts, others do receive rain, just not enough to compensate for high levels of evaporation. Our characters might express surprise about being rained on and not realize why the water doesn’t remain. Ten inches or less of rain causes a desert, while semi-deserts receive ten to twenty inches. If there’s grass, it’s called a steppe. A given desert may be one of these, but have portions of another type, especially if the desert is large. Since rainfall causes the differences, we should use our understanding of prevailing winds and rain shadows to determine what type of desert exists where. Land near a mountain range might be a desert while the area of that desert that is farthest from the mountain might turn into a steppe (because it’s getting more rainfall).
When it does rain in deserts, it’s often a violent downpour. This can cause flash floods to occur miles away from where the storm is.
A desert normally can’t form near the equator due to heavy rains in the tropics (from 0° to 30° latitude). However, high elevations (above 5,000 feet) cause a temperate climate to exist where we’d normally expect a tropical one. This can combine with a mountain range to form a rain shadow, resulting in a desert, such as those in Somalia.
Only 20% of deserts worldwide are sand, and in some areas, like North America, sand only covers 2%. The rest is a virtual pavement of tightly packed small stones (like pebbles) left after the dust has blown away. Once this happens, the ground is stable. Therefore, it’s not accurate to always state that humanoids or hooved animals are trudging slowly through sand.
Drifting sand accumulates as a series of sand dunes, or more rarely, a single dune. Extreme temperature changes from night to day cause rocks to break apart before further eroding occurs via wind, creating sand. Some deserts feature outcropping of rocks that help form an oasis when ground water remains. Established trade routes are often used to make crossing a desert less perilous. This is where we can add a settlement; most of these are likely to be small, not a megalopolis.
Hot deserts like the Sahara and Mojave have strong winds and low humidity. Cold deserts like the Gobi occur at higher latitudes and/or altitudes. Some deserts are farther inland because moist, warm air rises over the land closer to the sea and dumps rain there, leaving none for areas farther inland, causing a desert; this means we can have a forest between our desert and the sea. Some deserts are far from oceans or other large bodies of water, but others form to leeward of a mountain range, in its rain shadow, resulting in a long narrow desert near the coast. In other words, a north-to-south coast might have a north-to-south mountain range and then a north-to-south desert on the other side.
Sandstorms and dust storms aren’t the same thing. The dramatic images of towering storms are dust storms. Sand is too heavy to be lifted that high and doesn’t get much higher than a person when blown by the wind. It’s inaccurate to state the sand is falling on people from such a storm; dust is falling. Sand will pelt people in the body and face sideways, not from above. People can die from repeated exposure to dust storms when dust causes an incurable respiratory illness that ultimately causes them to suffocate.