You find these deserts on the western edge of continents near the Tropic of Cancer and the Tropic of Capricorn. Massive ocean water currents, think of them as rivers in the ocean, tend to move in a clockwise direction in the Northern Hemisphere and counter-clockwise in the Southern Hemisphere. Along the western edge of continents in both hemispheres the source of these currents tends to be found in the poles, so the oceans here are quite cold (the eastern edge of continents, by contrast, tend to be warm). Anyone who has swam in the warm waters off the Carolinas , and then went at the same latitude across North America to the frigid waters of California can prove this to themselves. Cold ocean currents inhibit the formation of rain cloud. Evaporation is a function of heat, so cold oceans do not provide the moisture that warm oceans do. Persistent high pressure systems are also a factor, tending to block incoming storms. Winter fog is often an important source of moisture in these coastal deserts, and very unusual plants and animals have developed to exploit this dew. The Atacama Desert of South America, and Mexico’s Baja California are examples.