R/calcFP.R

In ConSciR: Tools for Conservation Science

#' Calculate Frost Point
#'
#' @description
#' Function to calculate frost point (°C) from temperature (°C) and relative humidity (\%).
#'
#' @note
#' This function is unstable and is under development.
#'
#' @details
#' Formula coefficients from Arden Buck equation (1981, 1996) saturation vapor pressure over ice.
#'
#' \itemize{
#'   \item a = 6.1115
#'   \item b = 23.036
#'   \item c = 279.82
#'   \item d = 333.7
#' }
#'
#'
#'
#' @param Temp Temperature (°Celsius)
#' @param RH Relative Humidity (0-100\%)
#'
#' @returns Tf, Frost Point (°Celsius)
#' @export
#'
#' @examples
#' # calcFP is unstable and is under development
#' # Frost point at 20°C (Temp) and 50% relative humidity (RH)
#' calcFP(20, 50)
#' calcFP(0, 50)
#'
#'
#' # mydata file
#' filepath <- data_file_path("mydata.xlsx")
#' mydata <- readxl::read_excel(filepath, sheet = "mydata", n_max = 5)
#'
#' mydata |> dplyr::mutate(FrostPoint = calcFP(Temp, RH))
#'
#'
calcFP <- function(Temp, RH) {

  # Arden Buck (1981, 1996) coefficients for ice
  a_ice <- 6.1115
  b_ice <- 23.036
  c_ice <- 279.82
  d_ice <- 333.7

  # Partial vapour pressure
  Pws_ice <- a_ice * exp((b_ice - Temp / d_ice) * (Temp / (c_ice + Temp))) # Buck
  Pw_ice <- Pws_ice * RH / 100

  # Frost point (Tf)
  # Tf = (c_ice * log(Pw / a_ice)) / (b_ice - log(Pw / a_ice))
  Tf <- (c_ice * log(Pw_ice / a_ice)) / (b_ice - log(Pw_ice / a_ice))

  return(Tf)

}