Judging from the number of “confused” emoticon clicks in a recent thread about pollution in Chiang Mai, it seems many posters don’t understand what an inversion layer is……this is for those people:

 

Chiang Mai is one of many places around the world that is well known for its inversion layer, this is where meteorological conditions are frequently opposite to the way they should be. Under normal conditions, the air is hottest closest to the earth, it then becomes cooler the further you move into the atmosphere.

 

In an inversion layer, the opposite becomes true and a layer of cold air becomes trapped close to the ground so it cannot rise. The layer of cooler air is heavier and contains moisture hence it gives rise to fog, it also traps pollutants so PM levels frequently appear much higher than normal.

 

There is no denying that Northern Thailand has a burning problem and that farmers need to be educated and to change their way of working. But there are two other factors apart from local behaviour which impact poor air quality in Chiang Mai Province and these are largely outside of the control of the authorities, they include the inversion layer and the degree to which polluted air is blown in from surrounding countries.

 

There’s more about the subject of inversion layers on the web, some of it is here:

 

https://en.wikipedia.org/wiki/Inversion_(meteorology)

Also CM is not helped by being in a Bowl.

The Doi Saket and Suthep hills to East and West, Khun Tan hills to the south and the hills to the north before you reach Chang Dao.

 

john

You mention the inversion layer gives rise to fog but fog is rare in Chiang Mai. You would expect fog to be more common especially during the wetter times of the year but it isnt.

1 hour ago, CNXBKKMAN said:

You mention the inversion layer gives rise to fog but fog is rare in Chiang Mai. You would expect fog to be more common especially during the wetter times of the year but it isnt.

The distortion of the air we frequently see and think is polluted air is in fact fog in many cases, although it is often not very heavy and not always what we think of as fog - it remains that it is the same composition as fog, moisture in the air