Psychrometrics at altitude

Define the two air states

Both cases have a dry-bulb temperature of 75 °F and relative humidity of 50%. Only elevation changes. We estimate absolute station pressure from the standard atmosphere, using elevations of 0 ft and 5,000 ft. These are hypothetical engineering states, not observations of a particular city or day.

The calculation uses dry air plus water vapor, with no entrained liquid water. It is a comparison of two specified states; it does not model the path of an air parcel moving uphill.

Compare the calculated properties

Values are computed with the same calculation engine as the calculator, CLI, and MCP tools. Displayed values are rounded. Download CSV or JSON from either linked calculator case to retain unrounded results and the inputs.

1. Determine station pressure

In IP units, the standard-atmosphere approximation implemented by PsychroLib is:

P [psia] = 14.696 × (1 − 6.8754 × 10⁻⁶ × Z [ft])⁵·²⁵⁵⁹

At sea level, this gives 14.696 psia. At 5,000 ft, it gives approximately 12.228 psia. The reduction in total pressure changes the dry-air mass associated with a given air volume. When measured station pressure is available, enter it directly instead of using this estimate.

2. Resolve vapor pressure and humidity ratio

Relative humidity, expressed as a fraction φ, relates the water-vapor partial pressure to saturation vapor pressure at the dry-bulb temperature:

Pᵥ = φ × Pws(Tdb)
W = 0.621945 × Pᵥ / (P − Pᵥ)

For these two cases, dry bulb and relative humidity are identical, so vapor pressure is the same. Total pressure P is lower at 5,000 ft, reducing the denominator and increasing humidity ratio W. The same relative humidity therefore corresponds to more water per pound of dry air at altitude.

Dew point depends on water-vapor partial pressure. It remains the same in this comparison even though humidity ratio changes. Wet bulb also depends on total pressure, so it is not identical between the two cases.

3. Check enthalpy and specific volume

Using the IP relationships implemented by PsychroLib:

h [Btu/lb dry air] = 0.240 × Tdb [°F] + W × (1061 + 0.444 × Tdb [°F])
v [ft³/lb dry air] = 53.350 × (Tdb [°F] + 459.67)
                     × (1 + 1.607858 × W) / (144 × P [psia])

Substitute the unrounded humidity ratio and station pressure from either calculator export to reproduce its enthalpy and specific volume. More water vapor per pound of dry air increases enthalpy here. Lower total pressure increases specific volume: at 5,000 ft, it is approximately 20.5% higher than the sea-level value.

For a fixed actual volumetric flow at this state, dry-air mass flow is ṁda [lb/h] = 60 × CFM / v. Higher specific volume means less dry-air mass for the same actual CFM. Keep actual and standard volumetric flow distinct when evaluating fans, coils, and air processes.

Reproduce the example from the terminal

pipx install ventilatepro-cli
ventilatepro calc psychrometrics --dry-bulb 75 --relative-humidity 50 --elevation 0 --json
ventilatepro calc psychrometrics --dry-bulb 75 --relative-humidity 50 --elevation 5000 --json

These CLI calculations run locally without login. Use the installation guide for prerequisites and the MCP guide to use the same calculation through an AI client. Record the installed package version when saving a verification case.

Apply the comparison carefully

1. Use absolute local station pressure. A weather report’s sea-level-corrected pressure does not represent the actual pressure at an elevated site.
2. Keep mass bases and units explicit. Humidity ratio, specific enthalpy, and specific volume in this example use dry-air mass; moist-air density includes water vapor.
3. Evaluate both endpoints and airflow to determine an air-process load. A single state or this altitude comparison alone does not establish coil capacity.
4. Check design weather, equipment ratings, and governing project criteria separately. The standard atmosphere is an approximation, not a substitute for those inputs.

References and calculation provenance

Prepared by VentilatePro. Calculation basis: PsychroLib 2.5.0. The library documents its equations against the 2017 ASHRAE Handbook—Fundamentals. Library use does not imply ASHRAE endorsement.

Primary references: standard-atmosphere pressure, humidity ratio from vapor pressure, moist-air enthalpy, and specific volume.