The climate here reads well in two columns and badly in one. The rain column draws a very long dry season, from November to April, and a sharply marked wet one. The temperature column draws something else: a curve that rises exactly when the rain has not yet arrived. Anyone looking only at the first will choose badly.
01 The April trap
April's numbers: an average high of 35.3 degrees and an average low of 25.5. That is the year's maximum in both rows. And what you notice most is not the high but that low: when the night temperature does not drop below 25.5, the body gets no rest from the day's heat and tiredness accumulates from one day to the next.
02 Two months with almost no rain
The good window is short and it is very good. January totals 12.1 millimetres over 1.2 rainy days, with a high of 31.6 degrees, a low of 21.8 and 260 hours of sun. February drops to 6.6 millimetres over 1.1 days, the driest month of the year, though it already climbs to 33.2 degrees. December completes the trio with the year's lowest maximum, 30.8.
One rainy day a month is a figure worth translating. It means you can plan a whole day outdoors with a very high chance of getting it right, something that is simply impossible for the rest of the year. In a destination where almost everything interesting happens on the street or by the river, that predictability is worth more than a couple of degrees.
03 The monsoon and the river
The monsoon arrives in May and does not ease until November. In that stretch 1,127 of the annual 1,412 millimetres fall, and rainy days go from 6.8 in April to 15.9 in May, almost triple in one step. The city does not stop — tropical rain is intense and short — but predictability disappears entirely.
There is a second calendar that matters as much here as the sky's: the river's. The water level at the confluence follows the Mekong's flood with a lag behind the rain, so the river stays high when the sky has already cleared. If you are coming for the river landscape, November offers open sky and still-high water, which is the best combination of the two.
04 September, the month with the least sun
If April is the trap, September is the month that admits no argument. It totals 270.9 millimetres over 21.5 rainy days and only 129 hours of sun, the year's minimum by a wide margin; for comparison, March has 267. That is, September averages barely more than four hours of sun a day.
March has 267 hours of sun and September 129. It is the same destination with less than half the light.
It should be said, in fairness, that September and October are also when the plain is at its most spectacular, with the fields flooded and high water in every arm of the river. If your trip is about landscape rather than the city, those two months have a serious argument in their favour, provided you accept the closed sky.
05 The verdict
January, by a distance. It is the only month combining a low maximum, 31.6, the year's coolest minimum, 21.8, practically no rain and 260 hours of sun. December follows very closely and has an even lower maximum; February is drier still but already climbs to 33.2 with nights staying at 22.8.
And the advice that sums it up: read the minimum row before any other. The difference between January and April is 3.7 degrees of maximum, which sounds small, but also 3.7 of minimum, and that is what decides whether you sleep or not. The dry season here is not a uniform block: it has a good beginning and a hard end.
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