Moist-air state and psychrometric properties

Atmospheric Pressure

大气压力

Atmospheric pressure is the total pressure exerted by the surrounding atmosphere at a stated location and time.

Aliases: barometric pressure, ambient pressure

QUANTITY, SYMBOL AND UNIT

Quantity / objectAtmospheric pressure

Symbolp

SIPapascal

SIkPakilopascal

IPpsipound-force per square inch

Definition

Atmospheric pressure is the total pressure exerted by the surrounding atmosphere at a stated location and time.

In psychrometrics it is the total-pressure basis used with water-vapour partial pressure. Because pressure changes with altitude and weather, a chart or calculation prepared for one pressure may not represent another site exactly.

Relations and distinctions

Atmospheric and vapour pressure

Atmospheric pressure is the total mixture pressure; water-vapour partial pressure is only the contribution made by water vapour.

Actual and standard pressure

Actual local pressure is measured or obtained for the site and time. Standard-atmosphere pressure is a reference or estimate, not a substitute for every project condition.

Engineering meaning

State the pressure or site altitude when calculating humidity ratio, wet-bulb relationships or psychrometric processes.
Use a pressure basis consistent with the rating or test data when comparing equipment performance.

Common misconceptions

MisconceptionMoist-air calculations made at sea-level pressure apply unchanged at every altitude.

Accurate explanationPressure changes the relationship among vapour pressure, humidity ratio and other psychrometric properties, so the pressure basis must be consistent.

Application boundaries

Altitude-based standard-atmosphere equations estimate pressure but do not reproduce short-term weather variation.
Atmospheric pressure alone does not define temperature, humidity or moisture load.

Supporting sources

ASHRAE / SRC-ASHRAE-F25-CH01

ASHRAE Handbook - Fundamentals, Chapter 1: Psychrometrics ↗

2025 SI edition / Chapter 1, atmospheric-pressure, altitude and psychrometric-property sections
Supports: Atmospheric pressure is the total pressure used in moist-air property relationships; Atmospheric pressure varies with altitude and weather; Psychrometric calculations and charts require a stated pressure basis

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