Ha Long's best weather, spring's tail end into autumn's start: comfortable temperatures, less rain than the summer monsoon, and a break from winter's cold, clammy skies. Late October into November is the more reliable of the two windows, when the bay is calmest for cruising, April can still catch the very end of a cool spell or the first build of pre-monsoon heat. These are also the best months for clear photos of the karst islands, since haze and cloud cover thin out.

This tier covers real variety. December is markedly better than the months right after it, sunnier and less cool, though still firmly winter. January and February are the coldest, cloudiest, and most drizzle-prone months, with the least sunshine of the year. March starts warming but stays mostly grey. May and September sit at the edges of the summer monsoon, hot and increasingly wet but not yet, or no longer, at its worst.

The heart of the summer monsoon: hot, muggy, and prone to sudden heavy downpours, with August the wettest month of the year by a wide margin. Cruises can be affected by rough seas or cancelled routes when a typhoon passes through, typhoon season runs roughly late May to early November but peaks across these three months.