The year in the Chiang Mai valley
Temperature, rain and sunshine figures are the city's. The scoring does not come only from those figures: it takes in the burning season, which runs from December to April and which no climate table records.
Almost every method for choosing when to travel leans on the same three columns: temperature, rain and sunshine hours. It is a sensible method and it works in most destinations. In Chiang Mai it gives the opposite of the right answer.
The reason is that here the decisive variable appears in no climate table. It is air quality, and its worst moment coincides exactly with the best moment of the other three columns.
01 The month that wins on paper
Run through March's figures. Sunshine hours: 267.9, more than any other month of the year. Rain: 20.4 millimetres over 2.1 days. Average high: 35.4 degrees, still below April. Average low: 20.1, comfortable for sleeping.
If the criterion is clear skies and no rain, March wins comfortably. January follows with 258.4 hours and 11.3 millimetres, and February with 252.3 hours and 9.6 millimetres, which is the calendar's lowest rainfall figure.
And yet, of those three months, two are advisable and one is not at all. The difference between January and March is not on the thermometer or in the rain gauge: it is in what is suspended in the air.
02 What a climate table does not measure
Air pollution is a recurring environmental problem in Chiang Mai and it concentrates, year after year, between December and April. It coincides with the dry season because that is when stubble is cleared and burnt, and because in those months there is no rain to wash the air.
Geography makes it worse. The city sits in a valley ringed by mountains, and low-pressure systems arriving from the north trap forest-fire smoke between those mountains. What would disperse in open country stays put here.
One figure makes the scale clear. In 2018 PM2.5 particle concentration reached 183 micrograms per cubic metre, when the acceptable standard used by the World Health Organization was 25. And March is described as potentially the worst month of the year to visit the city precisely because of this.
For a traveller this has a very concrete and very counter-intuitive consequence. March's 267.9 hours of sunshine are hours of sun filtered through smoke: the sky is cloudless and the mountain opposite still cannot be seen. It is a clear month with no visibility.
03 The three real seasons
Setting the smoke aside, the year splits into three well-marked blocks. The cool, dry one runs from November to February: between 9.6 and 40.9 millimetres of rain a month, more than 229 hours of sunshine and average lows falling to 15.7 degrees in January.
The hot one runs from March to May and peaks in April, with average highs of 36.7 degrees and lows of 23.4. It is the only stretch of the year when heat alone dictates the day's plan, and it coincides with the end of the burning season.
And the wet one runs from June to October, peaking in August: 223.3 millimetres over 17.1 days and only 110.8 hours of sunshine, the annual minimum. Between the sunniest and the least sunny month there are 157 hours of difference, more than five hours a day.
04 The good window is short
Cross the three climate columns with the smoke calendar and the clean window narrows sharply: November, December and January. Three months, not the four or six usually quoted.
November is the most balanced of the three. It has rained 40.9 millimetres over 3 days, enough for the countryside to stay green after the wet season, there are 229 hours of sunshine and lows sit at 19.9 degrees. And the burning season has not yet begun.
December and January are the dry, cool months par excellence: 14.1 and 11.3 millimetres, 238 and 258.4 hours of sunshine, and average lows of 16.9 and 15.7 degrees. They are the only nights of the year when you need something warm in northern Thailand.
February sits in no man's land. Its rainfall figures are the year's best, but it is also when the air starts to load up, so it works at the start of the month and worsens as it goes. It is a month to check, not to book blind.
05 The rainy season is not what it looks like
The other prejudice, the opposite one, is worth dismantling. August piles up 223.3 millimetres, which sounds like a write-off, but it spreads them over 17.1 days, and in a tropical climate that means concentrated downpours, not whole days of continuous rain.
The real price of those months is not getting wet: it is the light. July and August stop at 115.3 and 110.8 hours of sunshine, less than half of March. The sky is covered most of the time and the landscape reads grey rather than colour.
In exchange, they are the months of clean air and green countryside, and also the cheapest. For anyone not coming to photograph distant mountains, October, with 123.8 millimetres over 9.1 days and 196.4 hours of sunshine, is a very defensible option.
The year's four stretches
From 40.9 down to 11.3 millimetres of rain a month, between 229 and 258.4 hours of sunshine and average lows falling from 19.9 to 15.7 degrees. Air still clean and countryside green in November. These are the three months when everything lines up.
The year's best figures — 9.6 and 20.4 millimetres, 252.3 and 267.9 hours of sunshine — coincide with the worst air. In 2018 PM2.5 reached 183 micrograms per cubic metre. April adds the calendar's highest average highs at 36.7 degrees.
Rain settles in with 167.5, 118.2 and 148.6 millimetres, and the smoke goes with it. Sunshine falls from 216.5 to 115.3 hours and lows sit at 24.5 degrees, the year's highest: humid heat and covered sky.
August and September are the wettest months, 223.3 and 207.2 millimetres, with the year's sunshine minimum in August: 110.8 hours. October turns towards dry with 123.8 millimetres and 196.4 hours of sunshine, and the countryside is at its greenest.
06 How to choose the month
The short rule is this: November if you can only pick one month, and December or January if November does not fit. All three have plenty of sun, next to no rain and cool nights, and none of them falls inside the bad part of the burning season.
The longer rule is more useful still: do not book March. Not even if the climate table says it is the sunniest month of the year. Especially if it says that, because that figure is precisely what hides the problem.
And if the trip falls in the wet season anyway, choose October over August: 123.8 millimetres against 223.3, 9.1 rainy days against 17.1 and 196.4 hours of sunshine against 110.8. It is the same green with almost twice the light.
