R/dewpoint.R

In chrras/climateeng: Tools to Calculate Climatic Comfort Related Parameters

dewpoint <- function(hum.ratio, alt = 0) {
  #' Dew-point temperature (t_d)
  #'
  #' Calculates the dew-point temperature based on humidity ratios.
  #' Eq. 37 & 38 - ASHRAE Fundamentals Handbook 2002, Psychrometrics.
  #' @param hum.ratio Vector of humidity ratios [kg/kg].
  #' @param alt Vector of altitudes [m]. Defaults to 0 m (sea level).
  #' @return Returns a vector of dew-point temperatures [degC].
  #' @export
  #' @examples
  #' dewpoint(hum.ratio = 0.010)
  #'
  #' hum.ratio = seq(0, 0.050, by = 0.001)
  #' plot(dewpoint(hum.ratio = hum.ratio), hum.ratio,
  #'      xlab = "Dew-point [degC]",
  #'      ylab = "Humidity ratio [kg/kg]")
  #' @author Christoffer Rasmussen

  par.w.press <- par_w_press(hum.ratio, alt)

  # Dewpoint temperature (eq. 37)
  dewpoint <- 6.54 + 14.526 * log(par.w.press) +
              0.7389 * log(par.w.press) ** 2 +
              0.09486 * log(par.w.press) ** 3 +
              0.4569 * par.w.press ** 0.1984

  dewpoint <- ifelse(dewpoint < 0,
                     6.09 + 12.608 * log(par.w.press) +
                     0.4959 * log(par.w.press) ** 2,
                     ifelse(dewpoint > 93, NA, dewpoint))

  return(dewpoint)
}