Imagine you are a merchant in the 15th century planning a trading expedition from the Swahili coast to India. You need to decide on the best time to set sail to ensure a safe and successful journey. Which factor is most critical to consider in your planning, based on your knowledge of Indian Ocean trade routes?

The most critical factor is the seasonal monsoon wind system, which determines when ships can catch the powerful winds that drive the voyage. In the Indian Ocean, the southwest monsoon blows from the southwest between April and June, pulling ships eastward from East Africa toward the Arabian Sea and India. If you sail too early or too late, the winds reverse, turning the journey into a slow, wind‑dead drift that risks starvation and damage. For instance, a trader leaving Mombasa in early May would ride the southwest monsoon, arriving in Calicut in a few weeks, whereas departing in November would leave the ship becalmed and vulnerable to storms. Thus, timing the departure to match the monsoon cycle is essential for a safe and timely expedition.