Moist-air state and psychrometric properties
Saturation
饱和状态
Saturation is the equilibrium moist-air state in which the water-vapour partial pressure equals the saturation pressure for the stated temperature and condensed phase.
- Aliases
- saturated moist air, saturation state
QUANTITY, SYMBOL AND UNIT
Quantity / objectSaturated moist-air state
UnitNot applicable
Definition
Saturation is the equilibrium moist-air state in which the water-vapour partial pressure equals the saturation pressure for the stated temperature and condensed phase.
For temperatures where liquid water is the reference, this is commonly described as 100% relative humidity. Below freezing, equilibrium over water and equilibrium over ice are different, so the phase basis must be stated.
Relations and distinctions
Saturation and condensation
Saturation describes an equilibrium boundary. Condensation is the formation of liquid water and also depends on surfaces, nucleation, time and local conditions.
Saturation over water and ice
The saturation vapour pressures over liquid water and ice are not identical below 0°C; dew-point and frost-point calculations use different branches.
Engineering meaning
- Use the correct water-or-ice saturation relationship in low-temperature calculations.
- Treat a near-saturation operating point as sensitive to sensor uncertainty, temperature gradients and cold surfaces.
Common misconceptions
MisconceptionA displayed 100% RH proves that every surface and location is uniformly saturated.
Accurate explanationThe reading represents one measurement and reference convention; local temperatures and sensor uncertainty can produce different surface conditions.
Application boundaries
- Supersaturation, droplets and fog require more context than the equilibrium definition alone.
- Saturation does not by itself quantify the moisture-generation rate or the equipment capacity required.
Supporting sources
-
ASHRAE / SRC-ASHRAE-F25-CH01
ASHRAE Handbook - Fundamentals, Chapter 1: Psychrometrics ↗
2025 SI edition / Chapter 1, saturation, dew-point and frost-point definitions
Supports: Saturation is an equilibrium boundary between moist air and a condensed water or ice phase; The saturation pressure depends on temperature and the selected condensed phase; Saturation over liquid water and saturation over ice are distinct below freezing
Maintenance record
Saturation
- Maintained by
- Yakeclimate Engineering Team
- Authoritative sources verified
- August 3, 2026
- Terminology checked
- August 3, 2026
- Last updated
- August 3, 2026
- Term ID
- EKS-HUM-044