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.
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.
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
calcPw(20, 50)
#> [1] 11.6917
# Calculate relative humidity at 50%RH
calcPw(20, 50) / calcPws(20) * 100
#> [1] 50
# 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))
#> # A tibble: 5 × 6
#> Site Sensor Date Temp RH Pw
#> <chr> <chr> <dttm> <dbl> <dbl> <dbl>
#> 1 London Room 1 2024-01-01 00:00:00 21.8 36.8 9.61
#> 2 London Room 1 2024-01-01 00:15:00 21.8 36.7 9.59
#> 3 London Room 1 2024-01-01 00:29:59 21.8 36.6 9.56
#> 4 London Room 1 2024-01-01 00:44:59 21.7 36.6 9.50
#> 5 London Room 1 2024-01-01 00:59:59 21.7 36.5 9.48
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"))
#> # 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 9.61 9.62 9.59 9.62
#> 2 London Room 1 2024-01-01 00:15:00 21.8 36.7 9.59 9.59 9.57 9.59
#> 3 London Room 1 2024-01-01 00:29:59 21.8 36.6 9.56 9.56 9.54 9.56
#> 4 London Room 1 2024-01-01 00:44:59 21.7 36.6 9.50 9.51 9.48 9.51
#> 5 London Room 1 2024-01-01 00:59:59 21.7 36.5 9.48 9.48 9.46 9.48