In addition to those notes, deserts and semi-arid environments also tend to form in the interiors of large landmasses. This is because even if there are no impediments like mountains to cause rainshadow... as moisture is moved across large land areas and cools, it condescends (rains). At some point, enough moisture has already been removed from the air that without a large body of water to draw more moisture from as air currents move further and further inland, the air gets drier and drier and the land gets less and less rainfall.

As I understand it, between rainshadow and the massive size of the Asian continent, this is what causes the Gobi Desert, which isn't very near the tropics at all.