Below the usual or expected level. In technical and scientific writing it is neutral and common: subnormal temperatures for July, subnormal rainfall, a subnormal white blood cell count. Weather reports, medical charts, and engineering papers use it exactly this way, and there is nothing unkind about it. But you must know the other history. For much of the twentieth century *subnormal* — and especially *mentally subnormal* or *educationally subnormal* — was the official term for people with intellectual disabilities; Britain even had 'ESN' schools. That use is now considered offensive and outdated, and because of it the word carries a bad smell when applied to a person's mind or ability. Some people will hear an insult even when none is meant. So the safe rule is: use *subnormal* for numbers, measurements, and conditions; never for a person. It is also a countable noun in that old, offensive sense, which is another reason to keep it to measurements. Note the grammar too — it sits before a noun most naturally (subnormal levels) and is a formal word; in conversation people just say 'lower than normal' or 'below average'.

低於正常的、偏低的（用於數值、溫度、指數；早期指智能障礙的用法在今日屬歧視語，勿用於人）