Why is ส in สวัสดี low tone but ส is a high tone consonant? I thought as it is a high sonorant syllable it would be a rising tone.

Because the gap between ส and ว before  ว is very short hence ส takes a low tone.

ว following ส takes a low tone here too because being next to ส, it follows the tonal rule of ส which is low tone here as it ends in ส

Another example of a consonant following the tonal rule of the former letter is ถนน

When น is next to น the tone is mid นนทบุรี for example but with ถนน the rising tone rule of ถ next to a soft vowel ( excuse my incorrect technical terms), forces นน to rise 

11 hours ago, transpose said:

Why is ส in สวัสดี low tone but ส is a high tone consonant?

 

It is not low tone.  It's mid tone in normal speech because the syllable is unstressed.  If you have a look at the Haas Thai Learners' Dictionary (available online at https://thai-notes.com/dictionaries/haas.html) you'll see it's pronounced /saˈwàtˈdii/.  Note it also doesn't have a glottal stop.  (For comparison, สระ with a stressed "a" is transcribed as /ˈsàʔ/ - same source.)

Thai people have a false view of how their language is pronounced, probably because they can't actually spell out a mid tone short vowel.  Unfortunately, almost all Thai teachers and learners' dictionaries get this wrong.  Material from AUA and the Union method do get it right.

Don't believe me? Let me quote a couple of authorities:

David Smyth in "Thai: An Essential Grammar"
“When the vowel in the first syllable is -a ... in normal speech the tone is mid”

 

James Higbie & Snee Thinsan in "Thai Reference Grammar"
“In spoken Thai, unstressed syllables in multisyllable words maybe given mid tones in stead of the high, low or rising tone they should have according to Thai spelling. In this book the common spoken form is used, and these syllables are given mid tones.”

Thank you for your replies. Yes, it sounds mid tone to me. I got the tone partly from http://thai-language.com/dict, saL watL deeM I suppose วั makes it a dead ending. On https://thai-notes.com/reading/tonerules.html, it says a high consonant live is rising and dead is low. Would you say then that this is over simplifying matters because it does not mention stress?

12 minutes ago, transpose said:

I suppose วั makes it a dead ending.

 

วั is not the end of a syllable and does not determine the tone of the syllable. It is written that way to indicate that there is a following consonant which can be live or dead and may affect the tone. 

 

If is written as วะ it is the end of a syllable, so no following consonant, and therefore has a dead ending.

 

Here is a graphic that has a chart of the tone rules.

 

 

 

13 hours ago, transpose said:

Thank you for your replies. Yes, it sounds mid tone to me. I got the tone partly from http://thai-language.com/dict, saL watL deeM I suppose วั makes it a dead ending. On https://thai-notes.com/reading/tonerules.html, it says a high consonant live is rising and dead is low. Would you say then that this is over simplifying matters because it does not mention stress?

 

It is oversimplifying.  That's why the site has a reading course at https://thai-notes.com/reading/index.html

 

The subject of unstressed "a" is covered in lessons 27 and 28.

 

That covers the tone of the first syllable.  Had you spotted that the tone of the second syllable is not what you'd expect?  วัส = low class consonant + short vowel + dead final.  According to the chart, that should be high tone.  Indeed, วัด, which follows the same pattern is high tone, /wát/.  However, in สวัสดี วัส is low tone. That's also covered in lesson 27.

I would suggest:

(1) You use a dictionary that gets the tones right.  I've previously linked to Haas.  The other one I would suggest is at https://thai-notes.com/dictionaries/predictionarysearch.html  but it does not include glottal stops in the transcription.  Wait until you can read Thai script fluently and accurately, so can ignore then transcription, before using the likes of thai-language.

 

(2) Work your way through the reading course (link above).  There are so many complexities and irregularities with written Thai, it would take an age to learn about all of them working piecemeal.

 

Finally, let me pick you up on one tiny thing:

 

On 3/2/2025 at 6:16 PM, transpose said:

Why is ส in สวัสดี low tone but ส is a high tone consonant?

ส is not a "high tone consonant" , it's a high class consonant.  There is no direct relationship between a consonant's class and the tone is produces.