EKS-HUM-007 / Moist-air state and psychrometric properties
Humidity Ratio
含湿量
Humidity ratio is the mass of water vapour in a moist-air sample divided by the mass of dry air in that sample.
- Abbreviations
- W
- Aliases
- mixing ratio, moisture content per dry air
QUANTITY, SYMBOL AND UNIT
Quantity / objectHumidity ratio
SymbolW
SIkg_w/kg_dakilogram of water vapour per kilogram of dry air
Commong_w/kg_dagram of water vapour per kilogram of dry air
Definition
Humidity ratio is the mass of water vapour in a moist-air sample divided by the mass of dry air in that sample.
It is commonly expressed as kg water/kg dry air or g water/kg dry air. Because sensible heating does not add or remove either dry air or water vapour, humidity ratio remains unchanged through an ideal sensible-heating process at fixed composition.
Formula and variables
W = m_w / m_da = 0.621945 p_w / (p - p_w) Conditions: The pressure form uses the perfect-gas relation for moist air. Total pressure p and water-vapour partial pressure p_w must use the same units.
| Symbol | Meaning | Unit |
|---|---|---|
W | humidity ratio | kg_w/kg_da |
m_w | mass of water vapour | kg |
m_da | mass of dry air | kg |
p_w | water-vapour partial pressure | Pa or kPa |
p | total moist-air pressure | Pa or kPa |
Relations and distinctions
Humidity ratio and specific humidity
Humidity ratio divides by dry-air mass; specific humidity divides by total moist-air mass.
Humidity ratio and RH
Humidity ratio is a mass ratio. RH is a temperature-dependent saturation ratio and can change during sensible heating while W remains constant.
Engineering meaning
- Use humidity-ratio differences with dry-air mass flow to calculate a steady moisture-removal rate.
- Include total pressure or altitude when deriving W from temperature and RH.
Common misconceptions
MisconceptionHumidity ratio and relative humidity are interchangeable.
Accurate explanationThey have different definitions, units and temperature behaviour; converting between them requires temperature and pressure.
Application boundaries
- The perfect-gas pressure formula is intended for ordinary moist-air calculations, not arbitrary high-pressure or non-air gas mixtures.
- Humidity ratio alone does not specify dry-bulb temperature, RH or equipment capacity.
Supporting sources
-
ASHRAE / SRC-ASHRAE-F25-CH01
ASHRAE Handbook - Fundamentals, Chapter 1: Psychrometrics ↗
2025 SI edition / Chapter 1, Basic Parameters and perfect-gas relationships, Equations 7, 8 and 21; lines 512–526 and 592–598
Supports: Humidity ratio is water-vapour mass per dry-air mass; Humidity ratio is related to vapour partial pressure and total pressure; The dry-air denominator differs from specific humidity's total moist-air denominator
Maintenance record
Humidity Ratio
- Maintained by
- Yakeclimate Engineering Team
- Authoritative sources verified
- August 3, 2026
- Terminology checked
- August 3, 2026
- Last updated
- August 3, 2026