The diagram below shows a simplified cross-section of the tri-cellular model of global atmospheric circulation, focusing on the Hadley and Ferrel cells.
With reference to the processes occurring at the boundary between the Hadley and Ferrel cells, explain how this global circulation system leads to the formation of arid climates (such as the Kalahari or the Sahara) at approximately 30∘ 30^\circ\,30∘ latitude.
