#' Satuated water vapour pressure (p_ws)
#'
#' Calculates the satuated water vapour pressure over water and ice.
#' Eq.5 & 6 - ASHRAE Fundamentals Handbook 2002, Psychrometrics.
#' @param temp.air Vector of air temperatures [degC].
#' @family PsychrometricGraph
#' @return Returns a vector of satuated water pressure over water and
#' ice [kPa].
#' @export
#' @examples
#' sat_w_press(temp.air = 25)
#' @author Christoffer Rasmussen
sat_w_press <- function(temp.air) {
temp.air <- temp.air + 273.15
# Coefficients for calculating the satuated pressure over water and ice
c <- c(-5674.5359, 6.3925247, -0.009677843, 6.22157E-07, 2.07478E-09,
-9.48402E-13, 4.1635019, -5800.2206, 1.3914993, -0.048640239,
4.17648E-05, -1.44521E-08, 6.5459673)
# Calculate satuated pressure over ice (Eq. 5) and water (Eq. 6)
sat.w.press <- ifelse(temp.air < 273.15,
exp(c[1] / temp.air +
c[2] +
c[3] * temp.air +
c[4] * temp.air ** 2 +
c[5] * temp.air ** 3 +
c[6] * temp.air ** 4 +
c[7] * log(temp.air)),
exp(c[8] / temp.air +
c[9] +
c[10] * temp.air +
c[11] * temp.air ** 2 +
c[12] * temp.air ** 3 +
c[13] * log(temp.air)))
# Return pressure in kPa
return(sat.w.press / 1000)
}
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