#' Converts specific humidity to vapour pressure.
#'
#' @param qair Required. Specific humidity (dimensionless).
#' @param psurf Required. Surface air pressure in Pa.
#'
#' @return If successful, returns the vapour pressure in kPa. If unsuccessful, returns the value \code{FALSE}.
#' @author Kevin Shook
#' @references R code for conversion of air pressure and absolute humidity was taken from project PEcAn The Predictive Ecosystem Analyzer \url{http://pecanproject.github.io}. The source code is available at \url{https://github.com/PecanProject/pecan/blob/master/modules/data.atmosphere/R/metutils.R.}
#' @export
#'
#' @examples ea <- qa2ea(0.001, 101325)
qa2ea <- function(qair, psurf){
# check parameter values
if (length(qair) == 0) {
cat('Error: missing humidities')
return(FALSE)
}
if (length(psurf) == 0) {
cat('Error: missing surface pressures')
return(FALSE)
}
psurf <- psurf * 0.01 # Pa -> mb
e <- qair * psurf / (0.378 * qair + 0.622)
ea <- e * 0.1 # convert mb back to kPa
return(ea)
}
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