calcPw: Calculate Water Vapour Pressure
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calcPw

R Documentation

Calculate Water Vapour Pressure

Description

Function to calculate water vapour pressure (hPa) from temperature (°C) and relative humidity (%).

Water vapour pressure is the pressure exerted by water vapour in a gas.

Usage

calcPw(Temp, RH, ...)

Arguments

`Temp`	Temperature (°Celsius)
`RH`	Relative Humidity (0-100%)
`...`	Additional arguments to supply to `calcPws`

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")

The water vapor pressure (P_w) is calculated using the following equation:

P_w=\frac{P_{ws}\left(Temp\right)\times RH}{100}

Where:

P_ws is the saturation vapor pressure using calcPws.
RH is the relative humidity in percent.
Temp is the temperature in degrees Celsius.

Value

Pw, Water Vapour Pressure (hPa)

Note

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

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

Examples

# Water vapour pressure at 20°C (Temp) and 50% relative humidity (RH)
calcPw(20, 50)

# Calculate relative humidity at 50%RH
calcPw(20, 50) / calcPws(20) * 100


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

mydata |> dplyr::mutate(Pw = calcPw(Temp, RH))

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

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