How is atmospheric pressure obtained from elevation?

From the U.S. Standard Atmosphere 1976 troposphere correlation P = 101325 × (1 − 2.25577×10⁻⁵ h)5.25588 Pa, where h is elevation in metres. Weather-report pressures are usually corrected to sea level and are not the actual local pressure.

Which saturation-pressure formula is used?

The ASHRAE (Hyland–Wexler) formulation over liquid water above 0 °C and over ice below 0 °C, valid roughly −100 to 200 °C. Dew point is obtained by numerically inverting the same relation.

What basis are enthalpy and specific volume on?

Per unit mass of dry air, as in ASHRAE psychrometrics: h = 1.006 t + W (2501 + 1.86 t) kJ/kg and v = 0.287042 T (1 + 1.607858 W) / P m³/kg.

Is this a substitute for measured site data or certified software?

No. It is a screening and educational tool. For contractual or safety-critical design use measured meteorological data, jurisdiction requirements and validated software.

Site Air Conditions Calculator — Atmospheric Pressure, Air Density, Psychrometrics

Site atmosphere and air properties

Ambient pressure and air properties drive most process, HVAC and combustion design. This tool derives barometric pressure and air density from elevation, full moist-air (psychrometric) properties from pressure, temperature and relative humidity, and the standard dry-air composition on both mole and mass bases.

Model assumptions

Barometric pressure uses the U.S. Standard Atmosphere 1976 troposphere correlation, accurate to roughly 11 km; real weather deviates from the standard atmosphere.
Moist-air properties follow the ASHRAE Handbook — Fundamentals psychrometric model (Hyland–Wexler saturation pressure), treating air as an ideal-gas mixture; enthalpy and volume are per unit mass of dry air.
Standard dry-air composition follows the CIPM-2007 reference; carbon dioxide is set from your input and the remaining species are renormalised while keeping their relative proportions.
All computation runs locally in your browser using SI base units; unit conversion is handled by the display layer.

References (provenance only)

U.S. Standard Atmosphere, 1976 (NOAA/NASA/USAF) — barometric pressure versus altitude.
ASHRAE Handbook — Fundamentals, Ch. 1 Psychrometrics; Hyland & Wexler (1983) saturation pressure.
Picard, Davis, Gläser & Fujii, Metrologia 45 (2008) — CIPM-2007 revised moist-air density formula and reference composition.

This tool does not encode inputs in the URL; share the page link only.

Frequently asked questions

How is atmospheric pressure obtained from elevation?: From the U.S. Standard Atmosphere 1976 troposphere correlation P = 101325 × (1 − 2.25577×10⁻⁵ h)^5.25588 Pa, where h is elevation in metres. Weather-report pressures are usually corrected to sea level and are not the actual local pressure.
Which saturation-pressure formula is used?: The ASHRAE (Hyland–Wexler) formulation over liquid water above 0 °C and over ice below 0 °C, valid roughly −100 to 200 °C. Dew point is obtained by numerically inverting the same relation.
What basis are enthalpy and specific volume on?: Per unit mass of dry air, as in ASHRAE psychrometrics: h = 1.006 t + W (2501 + 1.86 t) kJ/kg and v = 0.287042 T (1 + 1.607858 W) / P m³/kg.
Is this a substitute for measured site data or certified software?: No. It is a screening and educational tool. For contractual or safety-critical design use measured meteorological data, jurisdiction requirements and validated software.

Extended copy on this page (headings, assumptions, references, FAQs) may be drafted or localized with AI assistance; engineering judgment and governing codes still apply. Numerical models run locally in your browser as implemented. For contract-critical work, cite primary standards and qualified review.