Atmospheric circulation
The earth and ocean are intensively radiated in the equatorial areas. The hot and usually moist air rises. It is called convection. This process creates a low pressure area along the equator. The area is also called the Intertropical Convergence Zone. The hot air goes to both south and north and gets colder. When the air is cold enough, it goes down to the surface again. This creates deserts and semi-deserts of the subtropics.  Reaching latitude 30 degrees, the masses of air return to the rainy equator.