EKS-HUM-005 / Moist-air state and psychrometric properties

Relative Humidity

相对湿度

Relative humidity is the ratio of the actual water-vapour pressure in moist air to the saturation water-vapour pressure corresponding to the air temperature, expressed as a fraction or percentage.

Abbreviations: RH

QUANTITY, SYMBOL AND UNIT

Quantity / objectRelative humidity

Symbolφ, RH

SI1dimensionless ratio

Common%percent

Definition

Relative humidity is the ratio of the actual water-vapour pressure in moist air to the saturation water-vapour pressure corresponding to the air temperature, expressed as a fraction or percentage.

Because saturation pressure changes strongly with temperature, RH can rise or fall when air is heated or cooled even if no water vapour is added or removed. Temperature and measurement location therefore belong with every RH value used for an engineering decision.

Formula and variables

φ ≈ p_w / p_ws(t_db)

Conditions: Common perfect-gas form. Numerator and denominator must use the same temperature and compatible pressure basis; real-moist-air enhancement factors may be required outside ordinary conditions.

Symbol	Meaning	Unit
`φ`	relative humidity as a fraction	1
`p_w`	water-vapour partial pressure	Pa or kPa
`p_ws`	saturation pressure of pure water at dry-bulb temperature	Pa or kPa
`t_db`	dry-bulb temperature	°C

Relations and distinctions

RH and humidity ratio

RH is a temperature-dependent saturation ratio. Humidity ratio is water-vapour mass per unit mass of dry air and does not change under sensible heating at fixed pressure.

RH and dew point

Dew point tracks the saturation temperature associated with the existing vapour content; RH describes how close the current temperature is to saturation.

Engineering meaning

State dry-bulb temperature with RH when defining room, process or equipment inlet conditions.
Use calibrated sensors and consider local temperature gradients when an RH limit protects materials or equipment.

Common misconceptions

MisconceptionA fixed RH percentage always represents a fixed amount of water vapour.

Accurate explanationThe same RH at different temperatures represents different vapour pressures and moisture contents.

Application boundaries

A single RH sensor does not represent hidden cold zones, supply-air jets or conditions behind panels.
RH alone does not provide moisture generation, dehumidification capacity or condensation risk at an unknown surface temperature.

Supporting sources

ASHRAE / SRC-ASHRAE-F25-CH01

ASHRAE Handbook - Fundamentals, Chapter 1: Psychrometrics ↗

2025 SI edition / Chapter 1, Equations 13 and 23; lines 552–555 and 599–603
Supports: Relative humidity is a water-vapour-pressure ratio at the dry-bulb temperature; Relative humidity is dimensionless and commonly reported as percent; Relative humidity changes with temperature even when water-vapour content is unchanged
National Institute of Standards and Technology / SRC-NIST-HUMIDITY-CAL

Humidity Calibration Services at NIST ↗

Updated 2025-08-14 / Summary, description and calibration-uncertainty sections
Supports: Relative humidity is a calibrated humidity measurand whose result depends on temperature-controlled conditions

Maintenance record