Since the equator is the warmest part of our planet, more warm air occurs there. This warm air rises, and a combination of the rising air and the rotation of the planet causes winds. They move from east to west and sometimes are called easterlies or trade winds. These winds set up large cumulus clouds filled with moisture or, if over dry areas, they may pick up dust. Periodically they weaken a little. The reason is unknown, but this weakening may be involved in setting up the conditions for El Nino.