Throughout the Middle Islamic Period, especially during the Rasulid era (13th-15th centuries CE), navigation became an increasingly important mode of travel for trade and diplomacy between the Near East, Africa and Asia. One of the main ports along this major network was the Yemeni port of Aden, one of the best natural harbors in the world and a refuge for ships during rough seas. Details on the sailing seasons in and out of the port of Aden are provided in the Rasulid almanacs, but can also be pieced together from other sources, such as the travelers Ibn al-Muj?wir and Ibn Ba????a, as well as the account of the Chinese explorer Zheng He in the early 15th century. In this paper I will look at the role of the two major monsoon seasons in defining sailing seasons from the Red Sea through the Indian Ocean to the East African coast and Southeast Asia during the 12th-15th centuries. Given the vast extent of the sailing network, local climatic conditions must also be taken into account for analyzing sailing times. The monsoons, and winds in general, had a differential impact on travel in the Red Sea and Persian Gulf. It is also important to note that sea travel was also possible in local contexts without the aid of the monsoon winds. Given the variety of languages and peoples spoken, the sea route was a transcultural and border-crossing network par excellence, especially at a time with the turmoil following the Mongol invasion. A number of factors transcended geographical and political borders, including the need for a time reckoning system that could be understood across most of the network. This resulted in the use of a navigational nayr?z calendar (not to be confused with the Persian new year) described in Rasulid texts and later by Ibn M?jid. In addition to trade items, there was an exchange of information about sea conditions and navigation by use of star measurements.