Calculate Water Vapour Saturation Pressure

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) or August-Roche-Magnus approximation.

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 = "IAPWS")

Arguments

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

Value

Pws, Saturation vapor pressure (hPa)

Note

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.

Examples

calcPws(20)
#> [1] 23.39194

calcPws(20, method = "IAPWS")
#> [1] 23.39194
calcPws(20, method = "Magnus")
#> [1] 23.33441

# Calculate relative humidity at 50%RH
calcPw(20, 50) / calcPws(20, method = "IAPWS") * 100
#> [1] 50
calcPw(20, 50) / calcPws(20, method = "Magnus") * 100
#> [1] 50.12328

head(mydata) |> dplyr::mutate(Pws = calcPws(Temp))
#> # A tibble: 6 × 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
#> 6 London Room 1 2024-01-01 01:14:59  21.7  36.2  26.0