From Chinese 量 liàng (Cantonese loeng6).
Somehow I am puzzled that this does not have audio, would be really helpful to know the spelling
Check it here: https://forvo.com/word/lượng_mưa/
Is this any precipitation or rain specifically?
I'm curious too as "precipitation" isn't a word used that frequently in English.
I asked my native-speaking vietnamese father-in-law; he said "lượng mưa" sounds strange, but thinks it means more like rainfall or amount of rain in an area. He said normally, people just say "mưa" for rain.