Wind Patterns in the Indian Ocean

Winds are one of the most basic elements for navigation practice. The right time for a particular voyage to take place, or the right rhumb to follow at a particular time of the day would often depend on the winds that would be blowing at that time. Such was the case in 16th-century navigation practice all around the world.

In the case of the Indian Ocean, winds were perceived to follow a constant pattern every year, dividing the year into two long seasons. The northwest monsoon would take place from November to March and would be a favourable period to travel from southwest Asia to East Africa. Soon after it, between March and May, would start the southwest monsoon, giving rise to heavy precipitations. As Eric Staples put it in his Maritime Lexicon, this period is often considered “closed” for navigation, especially in the southwest and southeast Asia.

The type of winds within each monsoon varies also depending on the region and the time. The Arabic nautical rutters of the 16th century often offer alternatives depending on the kind wind that is blowing – “if the strong winds hit you,” they would say, “follow this direction, but if the wind is light, then, follow that.”

With the winds having such a strong influence on navigation, it is only natural that pilots themselves wondered at the origin and causes of such a geographical phenomenon. One of the famous pilots of the Indian Ocean, Sulaymān al-Mahrī (d. ca. 1550), dealt with this question in a small treatise called Tuḥfat al-Fuḥul (The Worthy Men’s Gift), written sometime between 1511 and 1554.

“Know” – he said – “that winds originate from air, for when the air moves there is an agitation.” One of the origins of such an agitation was coldness, and for that, al-Mahrī argued, there were many indications. One of them was that “if we are – for example – moving with a strong western wind, then rainy clous would originate from a direction other than west, and if the clouds come close to us and its cold reaches us, the first wind becomes calm – that is, the western wind – and a [second] wind comes from them (the clouds).”

Another indication, according to al-Mahrī, was that the land wind would only come at night, while the sea wind would come during the day. “And this is because of the coldness of the land at night and the heat of the sea in that time; and the oposite during the day, that is, the coldness of the sea and the heat of the land.”

Such was the theory of winds of al-Mahrī, full of echoes from and engaging in conversation with antiquity and medieval natural sciences. [Inês Bénard]