calcPws: Calculate Water Vapour Saturation Pressure

In ConSciR: Tools for Conservation Science

Calculate Water Vapour Saturation Pressure

Description

Function to calculate water vapour saturation pressure (hPa) from temperature (°C) using the International Association for the Properties of Water and Steam (IAPWS as default), Arden Buck equation (Buck), August-Roche-Magnus approximation (Magnus) or VAISALA conversion formula.

Water vapour saturation pressure is the maximum partial pressure of water vapour that can be present in gas at a given temperature.

Usage

calcPws(
  Temp,
  P_atm = 1013.25,
  method = c("Buck", "IAPWS", "Magnus", "VAISALA")
)

Arguments

Temp	Temperature (°Celsius)
P_atm	Atmospheric pressure = 1013.25 (hPa)
method	Character. Method to use for calculation. Options are "Buck" (default), "IAPWS", "Magnus" or "VAISALA".

Details

Different formulations for calculating water vapour pressure are available:

• Arden Buck equation ("Buck")

• International Association for the Properties of Water and Steam ("IAPWS")

• August-Roche-Magnus approximation ("Magnus")

• VAISALA humidity conversion formula ("VAISALA")

Value

Pws, Saturation vapor pressure (hPa)

Note

See Wikipedia for a discussion of the accuarcy of each approach: https://en.wikipedia.org/wiki/Vapour_pressure_of_water

If lower accuracy or a limited temperature range can be tolerated a simpler formula can be used for the water vapour saturation pressure over water (and over ice):

Pws = 6.116441 x 10^( (7.591386 x Temp) / (Temp + 240.7263) )

References

Wagner, W., & Pru\ß, A. (2002). The IAPWS formulation 1995 for the thermodynamic properties of ordinary water substance for general and scientific use. Journal of Physical and Chemical Reference Data, 31(2), 387-535.

Alduchov, O. A., and R. E. Eskridge, 1996: Improved Magnus' form approximation of saturation vapor pressure. J. Appl. Meteor., 35, 601-609.

Buck, A. L., 1981: New Equations for Computing Vapor Pressure and Enhancement Factor. J. Appl. Meteor. Climatol., 20, 1527–1532, https://doi.org/10.1175/1520-0450(1981)020<1527:NEFCVP>2.0.CO;2.

Buck (1996), Buck (1996), Buck Research CR-1A User's Manual, Appendix 1.

VAISALA. Humidity Conversions: Formulas and methods for calculating humidity parameters. Ref. B210973EN-O

See Also

calcMR for calculating mixing ratio

calcAD for calculating air density

calcPw for calculating water vapour pressure

calcPws for calculating water vapour saturation pressure

Examples

# Saturation vapour pressure at 20°C
calcPws(20)
calcPws(20, method = "Buck")
calcPws(20, method = "IAPWS")
calcPws(20, method = "Magnus")
calcPws(20, method = "VAISALA")

# Check of calculations of relative humidity at 50%RH
calcPw(20, 50, method = "Buck") / calcPws(20, method = "Buck") * 100
calcPw(20, 50, method = "IAPWS") / calcPws(20, method = "IAPWS") * 100
calcPw(20, 50, method = "Magnus") / calcPws(20, method = "Magnus") * 100
calcPw(20, 50, method = "VAISALA") / calcPws(20, method = "VAISALA") * 100


# mydata file
filepath <- data_file_path("mydata.xlsx")
mydata <- readxl::read_excel(filepath, sheet = "mydata", n_max = 5)

mydata |> dplyr::mutate(Pws = calcPws(Temp))

mydata |> dplyr::mutate(Buck = calcPws(Temp, method = "Buck"),
                              IAPWS = calcPws(Temp, method = "IAPWS"),
                              Magnus = calcPws(Temp, method = "Magnus"),
                              VAISALA = calcPws(Temp, method = "VAISALA"))

ConSciR documentation built on Nov. 5, 2025, 5:25 p.m.