EKS-HUM-008 / Moist-air state and psychrometric properties
Specific Humidity
比湿
Specific humidity is the mass of water vapour divided by the total mass of the moist-air sample.
- Aliases
- water-vapour mass fraction
QUANTITY, SYMBOL AND UNIT
Quantity / objectSpecific humidity
Symbolγ, q
SIkg_w/kg_makilogram of water vapour per kilogram of moist air
Commong_w/kg_magram of water vapour per kilogram of moist air
Definition
Specific humidity is the mass of water vapour divided by the total mass of the moist-air sample.
It is a mass fraction, commonly written as kg water/kg moist air or g water/kg moist air. Its value is slightly smaller than the humidity ratio for the same air state because the denominator includes both dry air and water vapour.
Formula and variables
γ = m_w / (m_da + m_w) = W / (1 + W) Conditions: All masses must use the same units. W is the humidity ratio on a dry-air mass basis.
| Symbol | Meaning | Unit |
|---|---|---|
γ | specific humidity | kg_w/kg_ma |
m_w | mass of water vapour | kg |
m_da | mass of dry air | kg |
W | humidity ratio | kg_w/kg_da |
Relations and distinctions
Specific humidity and humidity ratio
Specific humidity uses total moist-air mass; humidity ratio uses only dry-air mass. They are related but numerically different.
Specific humidity and vapour density
Specific humidity is mass per mass and is independent of sample volume; vapour density is mass per volume.
Engineering meaning
- Use specific humidity when a total moist-air mass fraction is required by a model or dataset.
- Confirm whether a source uses q, γ, mixing ratio or humidity ratio before combining data.
Common misconceptions
MisconceptionSpecific humidity is another name for humidity ratio.
Accurate explanationThe denominators differ, so the quantities and numerical values are not identical.
Application boundaries
- Symbol conventions vary by discipline; always state the definition and unit basis.
- Specific humidity alone does not specify temperature, saturation condition or condensation risk.
Supporting sources
-
ASHRAE / SRC-ASHRAE-F25-CH01
ASHRAE Handbook - Fundamentals, Chapter 1: Psychrometrics ↗
2025 SI edition / Chapter 1, Basic Parameters, Equations 9a and 9b; lines 520–526
Supports: Specific humidity is water-vapour mass divided by total moist-air mass; Specific humidity is related to humidity ratio by W/(1+W); The denominator differs from the dry-air basis of humidity ratio
Maintenance record
Specific Humidity
- Maintained by
- Yakeclimate Engineering Team
- Authoritative sources verified
- August 3, 2026
- Terminology checked
- August 3, 2026
- Last updated
- August 3, 2026