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While I was in Thailand in February (Bangkok, Chiang Mai, Krabi), I noticed low visibility throughout the country. It was like smog and it never got away. When flying I noticed this "smog" was instantly gone after reaching a couple kilometers of altitude.

Is this a normal phenomenom and what causes it?

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The haze in northern Thailand is indeed a yearly phenomenon, but the main driver for it is local farmers burning fields to prepare for planting in the following wet season. Since it's the dry season, there's nothing to contain or tamp down the smoke and dust, plus there's 'normal' air pollution from cities, cars etc in the region. Throw in the local topography — valleys surrounded on all sides by mountains, no open ocean nearby — and you get pretty bad haze.

Some of this obviously filters down all the way to Bangkok as well, although that city has pretty bad air all year around thanks to plain old smog. You were pretty unlucky to get hit in Krabi as well though.

This is all somewhat different from the Indonesia/Malaysia/Singapore haze, which is caused by (mostly illegal) jungle clearance. Having experienced both first hand, this style of haze is both much more unpredictable, as it depends on wind direction, and much worse on a bad day, since a burning jungle produces a lot more smoke than a few piles of rice straw on a paddy. Since most of this originates from Sumatra and Borneo, it's unusual for it to make it as far north as Thailand, but if Malaysia really gets walloped then the southernmost bits of Thailand may be impacted as well.

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    The main driver for the March haze is not farmers, but is forest fires burning in the region. Farmers burning the rice stubble contributes, as do locals burning off the winter fallen leaves, but forest fires are a more major contributor. Look at the NASA fire spotter website - firms.modaps.eosdis.nasa.gov/firemap/… You will notice that the hot spots are in the mountainous regions, not in the flats where farmers would be burning rice field stubble. Farmers get blamed because their fires are visible. – user13044 Mar 19 '15 at 18:00
  • FYI, for current Air Quality Index (AQI) readings in English and live photo in Chiang Mai, see Chiang Mai Air. For live 48-hour readings in English in various places in Thailand, see World Air Quality Index Project such as in Bangkok or Chiang Mai. – Basil Bourque Mar 16 '18 at 1:50
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While I can't be sure, I think this is a case of haze due to smoke from biomass burning.

There was an episode with a lot of burning events in February and March in Northern South-East Asia (the pictures are satellite pictures where active fires are highlighted). This has been reported in the region (news article, Singapore meteorological service), and is a well-known and annually recurring problem. Most of the fires are started by farmers to clear land, or for various other reason and tend to go out of control.

It also makes sense that the smoke will stay in lower altitudes (depending on weather). Of course this could be due to some other phenomenon that I am not aware of. If it was indeed smoke, then you might have noticed a burning smell in the air. Note that this seems to be a not so bad case and that visibility can get way worse.

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