<p>Crop burning is a major cause.  Crops are burnt Lal across Thailand between December and march and air movements bring large concentrations south to Bangkok.  Even today islands as remote as Koh Phangan are 135 on AQI.  Can't imagine tourism can work with crop burning.</p>

CC BY-SA 4.0
2020-12-24T03:50:43.983Z
Ok, but what is the mechanism?
peterh
CC BY-SA 4.0
2020-12-24T12:07:21.237Z
Given that sugar cane in Thailand is burnt between [November & March](https://www.channelnewsasia.com/news/asia/thailand-bangkok-air-pollution-smog-schools-shut-crop-burning-12296906), maybe there is an element of truth to this answer. This answer would be greatly improved if it had references/citations.
Fred
CC BY-SA 4.0
2020-12-24T01:23:50.530Z
2020-12-24T01:23:50.530Z
Jum
<p>I do not know about the specifics of Air Quality in Thailand and honestly most of my intuition comes from experience of moderate climates.</p>
<p>I would guess that the main reason for increase in frequency of pollution episodes is the slowing down of ground level atmospheric convection due to cooling of the air. This is a natural reason and probably the most important one.</p>
<p>However there is also an anthropogenic factor, namely an increase in fuel burning for heating. This definitely applies to Northern India and Central Asia and I would guess might apply to Thailand.</p>

CC BY-SA 4.0
2020-12-23T15:01:27.963Z
2020-12-23T15:01:27.963Z
Dima Chubarov
<p>For many locations this is typical.<br />
This is because the coldest weather in most places is typically caused by cold fronts, intrusions of cooler air from more poleward areas during the cool season.</p>
<p>If a cold front (which is an area of lower pressure) passes through, then high pressure will build in behind it.  High pressure tends to cause vertical setups called inversions, where air is cooler near than the surface than higher up in the atmosphere, preventing air from rising (see <a href="https://earthscience.stackexchange.com/questions/9302/why-are-inversion-layers-so-important-in-meteorology/9305#9305">Why Are Inversion Layers So Important In Meteorology</a>).  And near high pressures also tends to have lower pressure gradients, which means lower wind speeds.
Without any ability to rise or disperse outwards, pollution continues to build up until the weather pattern changes.</p>
<p>In these cold high pressures lower wind speeds and temperatures (plus the pleasant weather to work in, as well as the fact such cool periods can often be preceded by rain from the frontal passage) also tend to make good days for intentional burning (see <a href="https://www.auburn.edu/academic/forestry_wildlife/fire/weather_elements.htm" rel="nofollow noreferrer">this University of Auburn webpage</a>).</p>
<p>Plus in much of the world the cool season also brings falling debris from <a href="https://en.wikipedia.org/wiki/Deciduous" rel="nofollow noreferrer">deciduous</a> plants and leftover fields from harvests in the fall meaning it's a time when more people want to burn to clear out unwanted plants.<br />
Plus people may burn to keep warm.</p>
<p>So cool periods bring more benefits to burning, good conditions to do so, and weather conditions that cause the resulting pollution to build up.</p>
<p><strong>The question is: does Bangkok see such cold fronts ever?</strong></p>
<p>My familiarity is really only with weather patterns nearer the United States.  From that area, I might have guessed no, because the amount of fronts per year decreases as you get into areas like south Florida and parts of central America, which are at greater latitude than Bangkok.  But articles like <a href="https://www.thaipbsworld.com/cold-weather-has-arrived-with-temperatures-set-to-fall-further-in-coming-days/" rel="nofollow noreferrer">Cold weather has arrived with temperatures set to fall further in coming days</a> and <a href="https://www.thestar.com.my/news/nation/2019/12/16/chilly-mornings-to-continue-as-north-east-monsoon-brings-a-cold-front-says-met-dept" rel="nofollow noreferrer">Chilly mornings to continue as north-east monsoon brings a cold front, says Met dept</a> suggest it's a possibility.  It depends how properly the term cold front is being used in such articles, and what the exact meteorological circumstances are in them.</p>
<p>But perhaps the Himalayas\monsoon pattern indeed work together to allow cold air intrusions further south in some areas.  Someone more experienced with Malay\south Asian geography would need to give a more definitive word on this to have confidence.  But it appears, at the very least, cool enough conditions may occur to encourage burning and develop temperature inversions, which would lead to the same stagnant air much of the world sees when under high pressure.</p>

CC BY-SA 4.0
2022-06-20T03:07:15.850Z
Upon rereading this, it doesn't have to be a cold front as such.  Cool air in the wake of thunderstorms (which sometimes doesn't actually involve rainfall in a given location) and valleys with regular cloudcover due to the surrounding orography would both also favor increasing air pollution.
JeopardyTempest
CC BY-SA 4.0
2020-12-28T08:20:23.960Z
2020-12-28T08:29:41.823Z
2020-12-28T08:29:41.823Z
JeopardyTempest
<p>I reside in Bangkok where the local press have reported several times these two last months November-December about high levels of smog. A similar phenomenon occurred last year in this period (end of the tropical cold season); both this year and last year, when this smog was very evident I felt colder than in other days of the year.</p>
<p>Why does air pollution comes with cold weather in tropical areas?</p>

CC BY-SA 4.0
2020-12-25T17:36:29.110Z
you might want to check if this is a widespread phenomenon or just a local one.
John
<air-pollution>
Why does air pollution comes with cold weather in tropical areas?
anukmenchitlarmen
# Why does air pollution comes with cold weather in tropical areas?

(Asked by: username_0 on 2020-12-23)

I reside in Bangkok where the local press have reported several times these two last months November-December about high levels of smog. A similar phenomenon occurred last year in this period (end of the tropical cold season); both this year and last year, when this smog was very evident I felt colder than in other days of the year.

Why does air pollution comes with cold weather in tropical areas?

**Tags:** air-pollution

> **Comments:**
> **username_1**: you might want to check if this is a widespread phenomenon or just a local one.  

---

## 3 answers

### Answer by username_2 on 2020-12-24. Score: 2

Crop burning is a major cause. Crops are burnt Lal across Thailand between December and march and air movements bring large concentrations south to Bangkok. Even today islands as remote as Koh Phangan are 135 on AQI. Can't imagine tourism can work with crop burning.

> **Comments:**
> **username_3**: Ok, but what is the mechanism?  
> **username_4**: Given that sugar cane in Thailand is burnt between [November & March](https://www.channelnewsasia.com/news/asia/thailand-bangkok-air-pollution-smog-schools-shut-crop-burning-12296906), maybe there is an element of truth to this answer. This answer would be greatly improved if it had references/citations.  

---

### Answer by username_5 on 2020-12-23. Score: 0

I do not know about the specifics of Air Quality in Thailand and honestly most of my intuition comes from experience of moderate climates.

I would guess that the main reason for increase in frequency of pollution episodes is the slowing down of ground level atmospheric convection due to cooling of the air. This is a natural reason and probably the most important one.

However there is also an anthropogenic factor, namely an increase in fuel burning for heating. This definitely applies to Northern India and Central Asia and I would guess might apply to Thailand.


---

### Answer by username_6 on 2020-12-28. Score: 0

For many locations this is typical.  
This is because the coldest weather in most places is typically caused by cold fronts, intrusions of cooler air from more poleward areas during the cool season.

If a cold front (which is an area of lower pressure) passes through, then high pressure will build in behind it. High pressure tends to cause vertical setups called inversions, where air is cooler near than the surface than higher up in the atmosphere, preventing air from rising (see [Why Are Inversion Layers So Important In Meteorology](https://earthscience.stackexchange.com/questions/9302/why-are-inversion-layers-so-important-in-meteorology/9305#9305)). And near high pressures also tends to have lower pressure gradients, which means lower wind speeds.
Without any ability to rise or disperse outwards, pollution continues to build up until the weather pattern changes.

In these cold high pressures lower wind speeds and temperatures (plus the pleasant weather to work in, as well as the fact such cool periods can often be preceded by rain from the frontal passage) also tend to make good days for intentional burning (see [this University of Auburn webpage](https://www.auburn.edu/academic/forestry_wildlife/fire/weather_elements.htm)).

Plus in much of the world the cool season also brings falling debris from [deciduous](https://en.wikipedia.org/wiki/Deciduous) plants and leftover fields from harvests in the fall meaning it's a time when more people want to burn to clear out unwanted plants.  
Plus people may burn to keep warm.

So cool periods bring more benefits to burning, good conditions to do so, and weather conditions that cause the resulting pollution to build up.

**The question is: does Bangkok see such cold fronts ever?**

My familiarity is really only with weather patterns nearer the United States. From that area, I might have guessed no, because the amount of fronts per year decreases as you get into areas like south Florida and parts of central America, which are at greater latitude than Bangkok. But articles like [Cold weather has arrived with temperatures set to fall further in coming days](https://www.thaipbsworld.com/cold-weather-has-arrived-with-temperatures-set-to-fall-further-in-coming-days/) and [Chilly mornings to continue as north-east monsoon brings a cold front, says Met dept](https://www.thestar.com.my/news/nation/2019/12/16/chilly-mornings-to-continue-as-north-east-monsoon-brings-a-cold-front-says-met-dept) suggest it's a possibility. It depends how properly the term cold front is being used in such articles, and what the exact meteorological circumstances are in them.

But perhaps the Himalayas\monsoon pattern indeed work together to allow cold air intrusions further south in some areas. Someone more experienced with Malay\south Asian geography would need to give a more definitive word on this to have confidence. But it appears, at the very least, cool enough conditions may occur to encourage burning and develop temperature inversions, which would lead to the same stagnant air much of the world sees when under high pressure.

> **Comments:**
> **username_6**: Upon rereading this, it doesn't have to be a cold front as such.  Cool air in the wake of thunderstorms (which sometimes doesn't actually involve rainfall in a given location) and valleys with regular cloudcover due to the surrounding orography would both also favor increasing air pollution.  

---

