The band of storm clouds you see near the equator is caused by converging trade winds. This region is called the Intertropical Convergence Zone (ITCZ, a.k.a. "itch").
The trade winds are pushed westward by the Coriolis effect. In the Northern hemisphere, large wind patterns curve toward the right; in the Southern hemisphere, they curve left. You can watch this happen in an animation of NOAA satellite images here.
