The trade winds regularly sweep the Thousand Isles and are the lifeblood of maritime commerce in the region and key to the trading routes and strategies of the merchants. These winds blow reliably and quite strongly from Valtentis to Mbo, one way in spring, the opposite in fall. They stretch north to south from approximately the northern tip of Etmarch to the coast of northern Mbo
The Westerlies start in springtime (Westerly is a wind that originates in the West), generally around March, and blow until June, taking the great trade fleets from The Thousand Isles to Mythras and Valoria
The Easterlies generally start blowing in September and blow until December, taking the trade fleets home again from Valoria to The Thousand Isles.
Of course, there is also a counter cycle of trade, western trade fleets that follow the winds to The Thousand Isles to Mbo and back.
In high summer (between June and September) and high winter (December - March) the trades die away and winds often become unsettled and unpredictable, with long periods of complete calm (called The Doldrums), that risk stranding ships at sea.
In addition to the Trade Winds, the Isles are also home to great storms, like nothing Mythrans have experienced. NorWesters (also called Westerly Gales) can occur anytime of the year from winter till summer but are most common and severe in Springtime. These are cold, violent rainy storms, are not as severe as Typhoons but still fierce and deadly.
Around September, the region enters the Monsoon season, a period of intense rainfall that can last until December. The monsoons are frequently accompanied by Hurricanes (or Typhoons as they are sometimes called). These massive storms generally blow up from the southeast and sweep through the islands. Any ship that attempts to weather one of these storms at sea is sure to perish, so mariners generally ride them out in secure anchorages (called Hurricane Holes). The southern and eastern extents of The Thousand Isles are especially hard hit by these storms but they can wreak destruction all the way north to Gianthome. Typically the easterly trade fleet tries to return home early and beat the worst of the Typhoons, and generally sails via the most northerly route possible, skirting the northern edge of the trade winds, catching the tail of the Easterlies. But the timing of this is hard and it is not unheard of for an entire trade fleet to be lost to an early and strong typhoon.
see also: Climate of the Thousand Isles