Every continent has a dry strip between about 15º and 35º on its west coast, but they vary drastically in size:
- The Baja/Mojave and Atacama are fairly narrow, stretching about 300 miles inland at the widest (using the "25 cm annual precip" definition of desert.
- The Namib, about 600 miles.
- The Great Australian Desert covers about 2,000 miles, most of the continent.
- The Sahara, Arabian Desert, Dasht-e-Lut, and Baluchistan together stretch over 5,500!
These are all caused by the winds of the horse latitudes, in turn caused by the continental high-pressure zones, AKA subtropical ridges. But why does this produce just a slim strip of desert on some continents but a massive span southwest of Eurasia?
The size of the continent is the obvious answer, but if so why is Australia almost all desert, despite being the smallest continent, surrounded by warm ocean? Looking at the Andes, the Rocky Mountains, the mountains of Iran, Angola, and Namibia, and the Great Dividing Range, it appears that each subtropical desert tends to end at a mountain range, but not always: the southern Namib climbs half a mile before stretching across half of South Africa, and the Sahara has several highlands.