Relative Humidity

Relative humidity (φ, RH) is the ratio of the partial pressure of water vapor $p_w$ in the air to the saturation vapor pressure $p_{ws}$ at the same temperature:

$$\varphi = \frac{p_w}{p_{ws}} \times 100\ [\%]$$

Range of values

φ [%]	State
0	Perfectly dry air (theoretical)
30–60	Comfort zone for occupancy
100	Saturated air — at the point of condensation
> 100	Supersaturated — fog, rain

Dependence on temperature

Relative humidity depends on temperature because $p_{ws}$ rises with temperature:

• Heating at constant humidity ratio → φ falls
• Cooling at constant humidity ratio → φ rises; at φ = 100 % condensation begins

Example: air at 0 °C, φ = 80 % → heated to 20 °C, φ falls to about 21 %. That is why indoor air is so dry in winter.

Relative humidity in the Mollier diagram

In the h–x diagram, lines of constant relative humidity appear as curves sweeping across the diagram. The φ = 100 % line (the saturation curve) forms the boundary — beyond it (in the fog region below the curve) condensation would occur, and states do not exist there in equilibrium.

Measurement

• Capacitive sensor — the most common; measures the change in capacitance of a hygroscopic polymer
• Psychrometer (dry + wet bulb) — accurate, but requires calculation from the psychrometric equation
• Chilled-mirror hygrometer — the reference method for calibration

Comfort limits per standards

Standard	φ range	Note
Common design practice	30–60 %	Indicative comfort range for occupancy
ASHRAE 55-2023	no lower limit	Upper edge of the comfort zone set by humidity ratio x ≤ 0.012 kg/kg dry air, not by φ
ASHRAE 62.1	max. 65 %	Prevention of mold and dust mites