01 Less rain than Madrid, in the tropics
The distribution is what matters. January accumulates 175.2 mm over 14.2 rainy days; September, 5.2 mm over 0.9 days. That is a difference of thirty-four to one between the wettest and driest months, in a city where the temperature barely moves.
A note of honesty about the annual figure: the available series give between 779 and 910 millimetres depending on the period and the measuring station. All fall in the same band and all support the point, but no single figure should be trusted to the millimetre.
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02 Temperature is not a variable
The monthly mean runs from 26.1 to 28.3 degrees. Two degrees between the coolest month and the warmest, across the entire year.
Average highs never drop below 30.7 °C nor rise above 32.7. Lows move between 21.1 and 24.2. The heat record is 36.6 °C, in March, and the cold record 11.8 °C, in August, a surprisingly low figure for the tropics explained by the altitude inland.
What does genuinely change is humidity, and it tracks the rain: 81.5% in February against 67% in September. Those fourteen points are the difference between a sticky day and a dry one at the same temperature. If how it feels matters more to you than the number, choose by the humidity column.
Sunshine follows suit: some 2,828 hours a year, with September and October averaging 9.6 hours a day. And the sea stays between 27 and 30 °C all year, so swimming is never a matter of season.
03 Why the north coast is the dry one
Dili sits on the northern shore, and that shore is systematically drier than the southern one. In the wet season the north coast receives between 222 and 252 millimetres a month, falling to 12 or 18 in the dry. The south coast, by contrast, has a longer wet season, associated with the Australian monsoon.
The explanation lies in the relief: the island has a mountain spine reaching 2,963 metres that separates the two flanks. Systems arriving from the south unload on meeting that wall, and what crosses to the other side arrives wrung out. It is worth saying that the north-south contrast is well documented; the orographic mechanism is the obvious explanation, not a citation.
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04 October: the whales pass
There is an added reason to choose the end of the dry season. The Wetar Strait, east of Timor, is a corridor of very deep water connecting the Indian Ocean with the Pacific, and between October and December it becomes a major migratory route. It has been described as an ocean superhighway between the two oceans.
The species documented on that passage include blue whales, beaked whales, short-finned pilot whales and melon-headed whales, along with bottlenose and spotted dolphins. Dolphins and smaller cetaceans are seen year-round; the big passage is the autumn one. The strait's narrowest point measures 42.6 kilometres.
05 The window, and what you lose in it
The good window is long and clear: May to October, with its heart in August and September. August has 6.5 millimetres of rain, 275.9 hours of sun and the coolest nights of the year, lows of 21.1 °C. September drops to 5.2 millimetres and 67% humidity. October adds the cetacean passage and holds 272 hours of sun.
What you lose by choosing those months is the green. In August and September the inland landscape is parched: the eucalyptus have dropped their leaves, the riverbeds run dry and the slopes are brown. If what you want is the green island, you have to accept the rain and travel between January and April.
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One last nuance for anyone coming to dive: the dry season guarantees calm seas and reliable road access, which is what really determines whether boats go out. On underwater visibility the available sources do not agree and none is entirely solid, so the prudent approach is to count on the surface conditions — which are predictable — and ask the dive centres about conditions on the day.
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