What causes deserts to form?

Deserts form primarily due to a combination of climatic and geographical factors, with lack of rainfall being a key characteristic but not the sole cause. While low precipitation is essential for a region to be classified as a desert, the reasons behind this dryness vary and often involve complex interactions between atmospheric patterns, topography, and ocean currents. One major factor is the presence of high-pressure systems, especially around 30 degrees north and south of the equator, known as the subtropical highs. These areas experience descending dry air that inhibits cloud formation and precipitation, creating vast desert regions such as the Sahara and the Arabian Desert. Additionally, mountain ranges can cause a "rain shadow" effect, where moist air rises over a mountain, cools, and loses its moisture as rain on the windward side, leaving the leeward side dry and desert-like. The Great Basin Desert in the USA is an example of this. Other factors include cold ocean currents that cool the air above them, reducing its ability to hold moisture and leading to arid coastal deserts like the Atacama in South America. Furthermore, continental interiors far from moisture sources often become deserts due to the difficulty of moist air penetrating deep inland. In summary, deserts are caused by a combination of limited rainfall driven by atmospheric circulation patterns, topographical effects like rain shadows, and sometimes oceanic influences. Lack of rain is the defining feature, but the underlying geographical and climatic causes vary depending on the desert’s location.