EKS-HUM-009 / Moist-air state and psychrometric properties

Water-vapour Partial Pressure

水蒸气分压力

Water-vapour partial pressure is the pressure contribution assigned to water vapour within a moist-air gas mixture.

Aliases: water vapor partial pressure, vapour pressure in moist air

QUANTITY, SYMBOL AND UNIT

Quantity / objectWater-vapour partial pressure

Symbolp_w

SIPapascal

SIkPakilopascal

Definition

Water-vapour partial pressure is the pressure contribution assigned to water vapour within a moist-air gas mixture.

In the ordinary perfect-gas representation, the total pressure is the sum of dry-air and water-vapour partial pressures. Vapour partial pressure links RH, dew point, humidity ratio and saturation pressure without being identical to any of them.

Formula and variables

p = p_da + p_w

Conditions: Perfect-gas mixture relation for moist air; all pressures must use the same absolute-pressure units.

Symbol	Meaning	Unit
`p`	total pressure of moist air	Pa or kPa
`p_da`	partial pressure of dry air	Pa or kPa
`p_w`	partial pressure of water vapour	Pa or kPa

Relations and distinctions

Partial and saturation pressure

p_w describes the actual vapour in the air. p_ws(T) is the saturation pressure at a specified temperature.

Partial and total pressure

Water-vapour partial pressure is one component of the total moist-air pressure, not the whole barometric pressure.

Engineering meaning

Use p_w as the common bridge between RH, dew point and humidity-ratio calculations.
Keep all pressures on the same absolute-pressure unit and basis.

Common misconceptions

MisconceptionWater-vapour partial pressure is the same as relative humidity.

Accurate explanationPartial pressure has pressure units; RH is a dimensionless ratio comparing actual and saturation vapour pressures.

Application boundaries

The perfect-gas mixture relation is an engineering approximation whose domain must match the application.
Partial pressure alone does not state dry-bulb temperature or the surface temperature needed for condensation screening.

Supporting sources

ASHRAE / SRC-ASHRAE-F25-CH01

ASHRAE Handbook - Fundamentals, Chapter 1: Psychrometrics ↗

2025 SI edition / Chapter 1, perfect-gas relationships, Equations 15–21; lines 560–598
Supports: Moist air is represented as dry air and water vapour with component partial pressures; Total pressure is the sum of dry-air and water-vapour partial pressures in the perfect-gas model; Water-vapour partial pressure is related to humidity ratio

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