The idea that only after 1490 the European sailors had ‘come for the first time in recorded history to struggle with the limitations placed on sailing ships by the winds and currents of the open ocean’ does not correspond to reality. There is enough proof to show that much earlier, in the fifteenth century, the North-East Trades’ regularity, the wind variability in the zone directly north of them, and the Canaries Current were already known. When the fifteenth century ended, the Portuguese had already verified the symmetry of wind patterns in the Atlantic on both sides of the equatorial calm zone, which led them to apply the significant name of ‘ventos gerais’ to the trade winds of both hemispheres.

Accurate knowledge of the wind and current systems was essential to good navigation and the Portuguese ratters of the sixteenth century, chiefly ‘carreira da India’ rutters, include an increasing amount of information on that subject, referring especially to zones in the passage from the North Atlantic to the South Atlantic. The report of a voyage (1503) proves that at that time the Portuguese already knew the Gulf of Guinea winds and currents sufficiently well. As a result of oceanic sailing, the traditional ratters, exclusively coastal, developed considerably, not only by adding data about latitudes and compass variations but also by the indication of winds and currents. There appeared also an even newer type of ratter, the ‘oceanic rutter‘, in which the safest and quickest routes, because of the changeability of winds and current patterns, are indicated.

Information about elements concerning winds and currents which are included in the Portuguese ratters was revealed in printed matter after the end of the sixteenth century and this allowed some European scientists to study, because of the availability of better information, the causes of those physical phenomena, which had already been treated briefly in the sixteenth century by two nautical treatise writers, D. João de Castro and Father Fernando Oliveira.