Why do the vast majority of the world's deserts fall on the tropic of capricorn or cancer? I noticed on google maps today that the earth has two very distinct regions on which the majority of deserts are located. These fall on the tropic of capricorn and cancer. Illustrated here:



What causes this?
At thirty to thirty-eight degrees north and south of the equator are the so-called Horse Latitudes, regions with constant high-pressure systems. These regions inhibit precipitation. The phenomenon is quite well described and illustrated on Wikipedia:
https://en.wikipedia.org/wiki/Horse_latitudes