When a workshop humidity alarm goes off, the usual remark is “humidity is at 70%.” But 70% at 20 °C and 70% at 30 °C do not mean the same amount of water in the air, and in the same room the reading jumps as temperature changes between morning and evening. The reason is that “humidity” has at least two very different expressions: relative humidity and humidity ratio. Telling them apart is the first step in dehumidification, constant temperature and humidity control, and condensation prevention.
1. Relative humidity: how far the air is from being “full”
Relative humidity (symbol φ, unit %RH) is the ratio of the actual water vapour partial pressure to the saturation vapour pressure at the same temperature. Put simply, it tells how damp the air is right now, as a value from 0% to 100%; 100% means the air is saturated and cannot hold any more vapour.
The amount of vapour air can hold is not fixed: the warmer the air, the more it can hold; the colder, the less. Think of air as a stretchy bucket: temperature sets the size of the bucket, and the water vapour is the water inside.
An example calculated at standard atmospheric pressure: air at 30 °C and 50% RH contains about 13.3 g of vapour per kg of dry air, with a dew point of about 18.4 °C. If no vapour is added or removed and only the temperature falls, RH rises to about 66% at 25 °C, about 91% at 20 °C, and reaches 100% at about 18.4 °C, where condensation begins.
This is why fog forms and windows mist up after the temperature drops at night: no water was added, the “bucket” simply got smaller. Relative humidity therefore depends strongly on temperature; it reflects how saturated the air is, not how much water is really there.
2. Humidity ratio: the water that is actually in the air
Humidity ratio (also called moisture content; symbol d, unit g/kg dry air) is the mass of water vapour per kilogram of dry air. It is an absolute quantity that reflects the real amount