Winds were the driving force of ocean-going sail ships. Trade winds facilitated navigation, particularly in the Atlantic Ocean. They unquestionably played a decisive role in promoting the belief that the Indies could be reached by sailing west. Trade winds have a constant intensity and a steady direction; they blow from north-east to south-west in the northern hemisphere and from south-east to north-west in the southern hemisphere. Their range spans the entire zone between the Tropic of Cancer and the Tropic of Capricorn—in other words, the area traversed by the sailing routes between the Old and New Worlds.