for air moving parallel to the equator:
air moving to the east is moving more quickly than the earth beneath and is affected by a centrifugal force acting outward from the earth's axis of rotation that is stronger than the centrifugal force at the earth's surface -- this causes a deflection towards lower latitudes
air moving to the west is moving more slowly than the earth beneath and is affected by an outward-acting centrifugal force weaker than the centrifugal force on the earth's surface (which generates a force towards the axis of rotation in the direction of gravity) -- this causes a deflection toward higher latitudes