You eat too much fried food and the next morning you have a sore throat, a spot on your chin, an ulcer inside your lip. In English there is no word for what has happened to you. In Cantonese there is, everybody uses it, and it comes with an entire remedy system.
You have 熱氣 — hot air, heatiness. And you fix it by eating something cooling.
The scale is not about temperature. Ice cream is not cooling and hot soup is not heating. It is about what a food is understood to do to you — and the classification is stable enough that you can be handed a bowl of something with the explanation “you’ve been very 熱氣 lately” and know exactly what is being said about your recent behaviour.
What it’s for
Three things, mostly.
Diagnosis. Symptoms get sorted into the same system. Sore throat, mouth ulcers, spots, constipation, bad breath — heatiness. The cause is assumed to be recent and dietary, which means it is your own fault and also easily fixed.
Correction. Which is what herbal tea shops are for, and what the bitter melon in the fridge is for, and why somebody will put a flask of something in front of you without being asked.
Care, expressed sideways. This is the part outsiders miss. Telling someone they are 熱氣 and handing them a bowl is an act of attention — it means somebody has been watching what you eat and how you look. In a culture that doesn’t go in for a great deal of verbal affection, the soup is the sentence.
Is it Chinese or is it Cantonese?
Both, but not equally. Mandarin has 上火, which is in the neighbourhood and means roughly the same. What is distinctly Cantonese is the elaboration — the number of foods classified, the daily frequency of the vocabulary, the fact that a shop on every second corner exists to sell the correction, and that a Hong Kong child learns the scale long before learning anything about five phases or yin and yang.
The classical theory underneath all this is documented at hanzi.design. You do not need any of it to use the scale. Almost nobody who uses it could recite it.