How Chiang Mai reached single digit AQI, and why it didn't last

April 13, 2018
If you happened to be in Chiang Mai last weekend, then your lungs would have been treated to some spectacularly good air. Our network of sensors recorded the kind of numbers that would make even an Icelandic sheep take a deep cleansing breath. AQI's of 8 in downtown Chiang Mai, 7 in Mae Rim, and 4 in Chiang Dao. Yes, those are pm2.5 numbers.

Some of you might even have breathed a cautious sigh of relief that the smoky season was over for another year. Sadly not.

Let's look into the data to understand how we even got those kind of crazy low numbers last weekend, and how we ended up back over 150 so quickly.

Duh, it was windy!

Yes, it was windy, but that doesn't always guarantee a return to good air quality.

You might have noticed that sometimes even after an intense downpour and strong winds, the air quality barely changes, while other times the AQI plummets and we get some welcome relief from the unhealthy air. This is because wind events can occur over a large range of scales, from small localized gusts, to large thunderstorms, right up to the synoptic scale movement or air masses over hundreds of kilometers.

Beginning around noon on Saturday April 7th, Chiang Mai experienced a large-scale exchange of air mass as high pressure in the north forced a massive volume of cool air southward. This event was picked up by the weather station at Chiang Mai Airport and is highlighted in the wind speed and wind direction charts below.
Chiang Mai experienced 22 hours of wind speeds between 15 km/h and 39 km/h, and this type of sustained wind is how you can spot a large scale movement of air mass. Another indicator is consistent wind direction - in this case the wind was from due north (more on the significance of that wind direction in a moment).

This synoptic scale wind event is the main reason we were treated to some spectacular air quality. During Saturday afternoon our entire network of sensors spanning the entire city and beyond to Chiang Dao and Pai were all recording in the green. The chart below shows the time history of AQI recorded at Wat Ket, near the centre of town.

Contrast the impact this wind event had on air quality with another high wind even that happend around 5pm on April 11th. This is the tall bar to the right side of the wind speed plot above. This was a localized thunderstorm with intense but short-lived wind and rain. You can see that this weather event had very little impact on AQI below. Localized storms mix the air already in place rather than exchange it with cleaner air, hence the very rapid return to previous pollution conditions.

The eagled-eyed among you might have spotted a gap in the AQI plot below, right around the time of the thunderstorm. This was a result of the storm briefly knocking out power to our sensor!

A stroke of luck got us to single digit AQI

There was another factor that helped us reach such low AQI numbers last weekend, but to show it we need to enlist the help of some satellite data.

The map below shows data from NASA's Fire Information for Resource Management System (FIRMS). This data is provided by a Visible Infrared Imaging Radiometer Suite onboard the Suomi National Ploar-orbiting satellite. This sensor is able to identify differences in temperature between a "hot spot" or fire, and the surrounding land cover.

Each of the red squares in the image below represents a fire detected between April 7 and April 8, when we experienced the large scale movement of air mass from the north (Chiang Mai circled in black). By luck, there was very little upwind fire activity to the north during that time, which was the second factor which contributed to the very low AQI numbers.

Regional fires and slow moving air bring us back over 150

The wind speed chart above shows how windflow slowed significantly on Sunday evening and through the first half of the week. This, in combination with a big uptick in regional fire activity (shown below), brought us swiftly back to over 150 AQI by Wednesday.

Some takeaways

This last point is important to consider for those campaigning for better air in Chiang Mai. Forcing local government to acknowledge the problem and take steps to reduce our dangerous levels of air pollution is of course essential, but the problem is inherently a regional issue.

The final fire map above actually highlights how effective Northern Thailand has been in supressing fires compared to our regional neighbors. For instance, on April 09, total fires in our region detected by FIRMS were as follows:
There is still work to be done locally, but perhaps we should also be congratulating those efforts and encouraging the sharing of best practices with neighboring countries and more broadly within ASEAN.

We will be joining in the effort, and we would welcome your support. Our plan is to expand our citizen's network of sensors beyond Chiang Mai to help better understand the issue of cross-border pollution and how it impacts our own air here in Chiang Mai. If you live anywhere near our borders with Myanmar or Laos and would like to host a sensor, get in touch at


Weather data from Chiang Mai International Airport, Weather Underground
Wat Ket AQI,
Fire maps, NASA FIRMS, SE Asia centered map, additional technical info
Regional fire numbers: NASA