Four ways a desert forms
Deserts are defined by aridity, generally less than 250 mm of precipitation a year, but they arise through different mechanisms. Subtropical deserts such as the Sahara and the Arabian Desert sit near 30 degrees latitude under the descending, drying air of the Hadley cell. Rain-shadow deserts, like the Great Basin or the Atacama's eastern flank, form where mountains force air to drop its moisture on the windward side, leaving the leeward side dry.
Coastal deserts, including the Atacama and the Namib, occur where cold ocean currents chill the air and suppress rainfall while still producing fog. Polar deserts, such as much of Antarctica and the high Arctic, are arid because frigid air holds very little moisture, so precipitation is minimal despite the cold.
Same dryness, different causes
The diagram groups these by mechanism rather than by appearance. Two deserts can share the same low rainfall while owing it to entirely separate physics: atmospheric circulation, topography, ocean temperature, or extreme cold. The Atacama is often cited as one of the driest places on Earth, combining a rain shadow with a cold-current coastal effect, while polar deserts show that low temperature alone can produce desert conditions far from any sand dune.