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")
)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")
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
Examples
# Saturation vapour pressure at 20°C
calcPws(20)
#> [1] 23.3834
calcPws(20, method = "Buck")
#> [1] 23.3834
calcPws(20, method = "IAPWS")
#> [1] 23.39194
calcPws(20, method = "Magnus")
#> [1] 23.33441
calcPws(20, method = "VAISALA")
#> [1] 23.39249
# Check of calculations of relative humidity at 50%RH
calcPw(20, 50, method = "Buck") / calcPws(20, method = "Buck") * 100
#> [1] 50
calcPw(20, 50, method = "IAPWS") / calcPws(20, method = "IAPWS") * 100
#> [1] 50
calcPw(20, 50, method = "Magnus") / calcPws(20, method = "Magnus") * 100
#> [1] 50
calcPw(20, 50, method = "VAISALA") / calcPws(20, method = "VAISALA") * 100
#> [1] 50
# mydata file
filepath <- data_file_path("mydata.xlsx")
mydata <- readxl::read_excel(filepath, sheet = "mydata", n_max = 5)
mydata |> dplyr::mutate(Pws = calcPws(Temp))
#> # A tibble: 5 × 6
#> Site Sensor Date Temp RH Pws
#> <chr> <chr> <dttm> <dbl> <dbl> <dbl>
#> 1 London Room 1 2024-01-01 00:00:00 21.8 36.8 26.1
#> 2 London Room 1 2024-01-01 00:15:00 21.8 36.7 26.1
#> 3 London Room 1 2024-01-01 00:29:59 21.8 36.6 26.1
#> 4 London Room 1 2024-01-01 00:44:59 21.7 36.6 26.0
#> 5 London Room 1 2024-01-01 00:59:59 21.7 36.5 26.0
mydata |> dplyr::mutate(Buck = calcPws(Temp, method = "Buck"),
IAPWS = calcPws(Temp, method = "IAPWS"),
Magnus = calcPws(Temp, method = "Magnus"),
VAISALA = calcPws(Temp, method = "VAISALA"))
#> # A tibble: 5 × 9
#> Site Sensor Date Temp RH Buck IAPWS Magnus VAISALA
#> <chr> <chr> <dttm> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl>
#> 1 London Room 1 2024-01-01 00:00:00 21.8 36.8 26.1 26.1 26.1 26.1
#> 2 London Room 1 2024-01-01 00:15:00 21.8 36.7 26.1 26.1 26.1 26.1
#> 3 London Room 1 2024-01-01 00:29:59 21.8 36.6 26.1 26.1 26.1 26.1
#> 4 London Room 1 2024-01-01 00:44:59 21.7 36.6 26.0 26.0 25.9 26.0
#> 5 London Room 1 2024-01-01 00:59:59 21.7 36.5 26.0 26.0 25.9 26.0