The latitudinal explanation, however, is only partial. Around the mid-latitudinal belt, only the western side of continents is normally occupied by deserts, while the eastern side is covered by forests. The reason for this has to do with the global circulation of ocean currents: gravitation from the sun and the moon pulls air and water on the earth's surface and tends to make them lag behind, relative to the earth's rotational movement.

The gravitational drag is greatest in the equator, where the centrifugal speed of the earth is fastest. Thus, as the earth turns, ocean currents and winds flow in the equator from east to west, tugged by universal gravitation, forming the equatorial currents and the easterly trade winds. As the westbound surface waters move away from the continents, they pull cold, nutrient-rich waters to the surface that generate a cool, stable coastal atmosphere, with little evaporation from the sea and very low rainfall other than morning fogs (Figure 1.5). In the coasts neighbouring these oceanic upwellings, typical coastal fog deserts tend to develop, forming some of the driest ecosystems on earth. Thus, the largescale circulation of the ocean is the main reason why coastal deserts are always found on the west side of continents, such as the Namib in Africa (Figure 1.6), Atacama in Chile, the Atlantic Coastal Desert of Morocco, or the deserts of Baja California (Figure 1.7).