01 Two figures that explain everything
The annual mean is 28.1 °C, with mean highs of 34.8 and lows of 22.2. Rain falls on 17.7 days a year and is concentrated in winter: December, January, February and March account for more than 80% of the total. In June and September the record shows zero millimetres.
The most surprising detail in the table is not the heat, which you expect, but the cold: the city's record low is 5.4 °C, measured in February. In a Gulf capital, that means winter nights can call for a jacket.
02 Summer is not negotiable
August is the hottest month: a mean high of 43.4 °C and a daily mean of 35.9. July follows with highs of 42.5, and holds the series record: 49.0 °C. June sits at 41.4 and September at 40.9.
In July the mean low is 29.7 degrees. It is not that the days are hot: the night never gets round to cooling.
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It might seem that with air conditioning none of this matters, and in part that is true: the city is set up to function indoors twelve months a year. But any plan involving the open air — the mangroves, the beach, a site visit, a walk along the corniche — stops being viable between May and September.
03 Humidity runs backwards
This is the figure that ruins most plans, because it is counterintuitive. The year's highest mean relative humidity is not in summer: it is in December (69%) and January (68%). The lowest is in May (50%) and April (53%).
That said, relative humidity is measured against air at whatever temperature it happens to be. 54% at 43 degrees in August holds far more water than 69% at 21 degrees in December. That is why September, at 60% with highs of nearly 41, is in practice the year's most uncomfortable month, even though it is not the hottest.
04 The good window
November to March, and there is no argument. January is the coolest month, with a mean of 19.1 °C and highs of 24.5; February rises to 20.6 and March to 23.4. November and December sit at 25.2 and 21.1. Those are five months of European-spring temperatures with sunshine practically guaranteed.
The little rain there is falls then: March is the wettest month of the year with 12.9 millimetres over 2.9 days, and January follows with 12.5. These are trivial amounts in absolute terms, but worth knowing, because in a city whose drainage was not designed for water, a ten-millimetre downpour shows on the street.
And if you have no choice but to come in summer, invert the day. Be out before eight, keep midday and the afternoon for whatever is under a roof — and here that is a great deal — and go back outside from seven. It is the only way to keep the trip from turning into a tour of air-conditioned interiors.
G Da ToledoIt only rains fifty-three days a year here.
26D TashkentAugust brings two millimetres of rain and February seventy-two.
Vyacheslav Argenberg Hanoi