What does air humidity mean? What is the highest humidity?

Air humidity (absolute humidity) is the total mass of water molecules in the air, and the unit is mg/m3.

The maximum humidity is meaningful only after the temperature and air pressure are reached for the first time.

Under certain temperature and pressure conditions, the maximum humidity is the maximum of the total mass of water molecules that air can carry under sub-conditions.

The air humidity mentioned in the weather forecast is relative humidity, expressed as a percentage. That is, under certain temperature and pressure conditions, the total mass of water molecules actually contained in the air is divided by the maximum value under this condition, and the score is converted into percentage.