|Photo by Vinoth Chandar|
For navigators, the term Monsoon refers to a seasonal wind.
This usage is reflected in the roots. According to Wordnik: ", monsoon comes from the Arabic word " mawsim" which means season. Obsolete Dutch monssoen, from Portuguese monção, from Arabic mawsim, season, from wasama, to mark; see wsm in Semitic roots."
Essentially monsoons are a large scale, continental version of sea and land breezes which occur on the coast. The driving force behind both monsoons and sea breezes is the different rates at which sea and land heat and cool. Land heats and cools at a faster rate then the sea. Warm air rises and is replaced by cooler air. Thus the daily (in the case of sea breezes) and seasonal (in the case of monsoons) reversals of wind direction.
The most well known Monsoon is the Indian Monsoon. From May to September, the winds are SW, from sea to land. From December to February winds are NE, from land to sea.
The summer Monsoon famously brings moisture from the sea to the Indian sub-continent which results in (sometimes heavy) rain. This accounts for fact that the term Monsoon is often used to refer to the rainy season only (or sometime just to refer to heavy rain).
For navigators, the less well know, but equally important winter Monsoon brings NE winds.
In practical terms the Monsoons influence the weather over a very large area. On a voyage from the Suez Canal to Japan, the effects of the SW monsoon are first felt in the southern Red Sea but more dramatically upon leaving the Gulf of Aden and entering the Arabian sea. There, exposed to long fetch of the Indian Ocean, the SW monsoon winds sometimes result in a heavy swell. Entering the Straits of Malacca the weather is dominated by local conditions but after leaving Singapore Straits and proceeding northward in the South China Sea the monsoon reasserts itself.
The term monsoon is used to refer to any seasonal reversal of winds - here is an article about the Arizona or Mexican Monsoon.