The Indian Ocean - Michael Pearson [16]
It was these winds which very largely determined when people could sail where. The monsoon winds were absolutely vital, even if Felipe Fernández-Armesto was putting it a bit strongly when he wrote that
Throughout the age of sail – that is, for almost the whole of history – wind determined what man could do at sea: by comparison, culture, ideas, individual genius or charisma, economic forces and all the other motors of history meant little. In most of our traditional explanations of what has happened in history there is too much hot air and not enough wind.20
There are some regional specificities and details to consider, these acting to complicate the simple pattern outlined above, and also to put a premium on experience and knowledge. The pattern of winds in the Arabian Sea is familiar enough. Many authorities stress the divide of the Swahili coast at Cape Delgado, which is just south of the mouth of the Ruvuma River, which river forms the boundary today between Tanzania and Mozambique. As a rule of thumb, down to Cape Delgado is one monsoon from Arabia and India, but south of there is two. Here then we see a deep structure element, the monsoons, privileging the northern Swahili coast, for it was more accessible to centres in India and Arabia than was the south.
The northeast monsoon starts in November and one can leave the Arabian coast at this time and reach at least Mogadishu. However, the eastern Arabian sea has violent tropical storms in October and November, so for a voyage from India to the coast it was best to leave in December, by which time the northeast monsoon was well established as far south as Zanzibar: a rapid passage of twenty to twenty-five days could be expected. By March the northeast monsoon was beginning to break up in the south, and by April the prevailing wind was from the southwest. This was the season for sailing from the coast to the north and east. At its height, in June and July, the weather was too stormy, so ships departed either as this monsoon built up in May, or at its tail end in August. An important general point here is that both monsoons prevailed longer the further north on the coast one was. In the far south we are really outside the monsoon system. In particular, the southwest monsoon is not nearly as strong and predictable as it is further north in the monsoon zone. Up to Mozambique Island there was really no monsoon, and indeed some would claim that the notion of a monsoon system really only applies in the northern hemisphere, or at most to about 10° S.
Moving around to the Red Sea area and southern Arabia, there were other particular things to take account of. An English traveller in 1780 wrote of the pattern in and around the Red Sea:
As different winds prevail on the different sides of the Tropic in the Red Sea, ships may come to Gedda [Jiddah] from opposite points at the same season of the year; those which come from Suez at the above mentioned time [that is, November to January], benefit by the N.W. wind, while those that come from India and Arabia Felix are assisted by the regular S.W. monsoon. The pilgrims... embark at Gedda time enough to avail themselves of the Khumseen [according to Capper this is Arabic for 50, which is the length of time this wind blows] wind, which blows southerly from the end of March to the middle of May, and conveys them in less than a month back again to Suez; the India vessels must also quit Gedda so as to be out of the straits of Babelmandel before the end of August.21
Even today within the Red Sea the monsoons act as a governing factor for traditional navigators, as a modern account of the sea's routes, winds and sailing times makes clear.22
This situation of course pertained even more strongly concerning the traffic between the Red Sea and western India. In the great fifteenth-century trade between Calicut and the Red Sea, ships left Calicut in January, and vessels from the Red