Description Usage Arguments Details Value Note References Examples
Approximate water pressure in air as a function of temperature, and its inverse the calculation of dewpoint.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32  water_vp_sat(
temperature,
over.ice = FALSE,
method = "tetens",
check.range = TRUE
)
water_dp(water.vp, over.ice = FALSE, method = "tetens", check.range = TRUE)
water_fp(water.vp, over.ice = TRUE, method = "tetens", check.range = TRUE)
water_vp2mvc(water.vp, temperature)
water_mvc2vp(water.mvc, temperature)
water_vp2RH(
water.vp,
temperature,
over.ice = FALSE,
method = "tetens",
pc = TRUE,
check.range = TRUE
)
water_RH2vp(
relative.humidity,
temperature,
over.ice = FALSE,
method = "tetens",
pc = TRUE,
check.range = TRUE
)

temperature 
numeric vector of air temperatures (C). 
over.ice 
logical Is the estimate for equilibrium with liquid water or with ice. 
method 
character Currently "tetens", modified "magnus", "wexler" and "goff.gratch" equations are supported. 
check.range 
logical Flag indicating whether to check or not that
arguments for temperature are within the range of validity of the

water.vp 
numeric vector of water vapour pressure in air (Pa). 
water.mvc 
numeric vector of water vapour concnetration as mass per volume (g m3). 
pc 
logical flag for result returned as percent or not. 
relative.humidity 
numeric Relative humedity as fraction of 1. 
Function water_vp_sat()
provides implementations of several
well known equations for the estimation of saturation vapor pressure in
air. Functions water_dp()
and water_fp()
use the inverse of
these equations to compute the dew point or frost point from water vapour
pressure in air. The inverse functions are either analytical solutions or
fitted approximations. None of these functions are solved numerically by
iteration.
Method "tetens"
implements Tetens' (1930) equation for the cases of
equilibrium with a water and an ice surface. Method "magnus"
implements the modified Magnus equations of Alduchov and Eskridge (1996,
eqs. 21 and 23). Method "wexler"
implements the equations proposed
by Wexler (1976, 1977), and their inverse according to Hardy (1998). Method
"goff.gratch"
implements the equations of Groff and Gratch (1946)
with the minor updates of Groff (1956).
The equations are approximations, and in spite of their different names,
Tetens' and Magnus' equations have the same form with the only difference
in the values of the parameters. However, the modified Magnus equation is
more accurate as Tetens equation suffers from some bias errors at extreme
low temperatures (< 40 C). In contrast Magnus equations with recently
fitted values for the parameters are usable for temperatures from 80 C to
+50 C over water and 80 C to 0 C over ice. The Groff Gratch equation is
more complex and is frequently used as a reference in comparison as it is
considered reliable over a broad range of temperatures. Wexler's equations
are computationally simpler and fitted to relatively recent data. There is
little difference at temperatures in the range 20 C to +50 C, and
differences become large at extreme temperatures. Temperatures outside the
range where estimations are highly reliable for each equation return
NA
, unless extrapolation is enabled by passing FALSE
as
argument to parameter check.range
.
The switch between equations for ice or water cannot be based on
air temperature, as it depends on the presence or not of a surface of
liquid water. It must be set by passing an argument to parameter
over.ice
which defaults to FALSE
.
Tetens equation is still very frequently used, and is for example the one recommended by FAO for computing potential evapotranspiration. For this reason it is used as default here.
A numeric vector of partial pressures in pascal (P) for
water_vp_sat
and water_mvc2vp
, a numeric vector of dew point
temperatures (C) for water_dp
and numeric vector of mass per volume
concentrations (g m3) for water_vp2mvc
.
The inverse of the Groff Gratch equation has yet to be implemented.
Tetens, O., 1930. Uber einige meteorologische Begriffe. Zeitschrift fur Geophysik, Vol. 6:297.
Goff, J. A., and S. Gratch (1946) Lowpressure properties of water from 160 to 212 F, in Transactions of the American Society of Heating and Ventilating Engineers, pp 95122, presented at the 52nd annual meeting of the American Society of Heating and Ventilating Engineers, New York, 1946.
Wexler, A. (1976) Vapor Pressure Formulation for Water in Range 0 to 100°C. A Revision, Journal of Research ofthe National Bureau of Standards: A. Physics and Chemistry, SeptemberDecember 1976, Vol. 80A, Nos.5 and 6, 775785
Wexler, A., Vapor Pressure Formulation for Ice, Journal of Research of the National Bureau of Standards  A. Physics and Chemistry, January  February 1977, Vol. 81A, No. 1, 519
Alduchov, O. A., Eskridge, R. E., 1996. Improved Magnus Form Approximation of Saturation Vapor Pressure. Journal of Applied Meteorology, 35: 601609 .
Hardy, Bob (1998) ITS90 formulations for vapor pressure, frostpoint temperature, dewpoint temperature, andenhancement factors in the range 100 TO +100 C. The Proceedings of the Third International Symposium on Humidity & Moisture, Teddington, London, England, April 1998. http://www.decatur.de/javascript/dew/resources/its90formulas.pdf
Monteith, J., Unsworth, M. (2008) Principles of Environmental Physics. Academic Press, Amsterdam.
[Equations describing the physical properties of moist air](http://www.conservationphysics.org/atmcalc/atmoclc2.pdf)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34  water_vp_sat(20) # C > Pa
water_vp_sat(temperature = c(0, 10, 20, 30, 40)) # C > Pa
water_vp_sat(temperature = 10) # over water!!
water_vp_sat(temperature = 10, over.ice = TRUE)
water_vp_sat(temperature = 20) / 100 # C > mbar
water_vp_sat(temperature = 20, method = "magnus") # C > Pa
water_vp_sat(temperature = 20, method = "tetens") # C > Pa
water_vp_sat(temperature = 20, method = "wexler") # C > Pa
water_vp_sat(temperature = 20, method = "goff.gratch") # C > Pa
water_vp_sat(temperature = 20, over.ice = TRUE, method = "magnus") # C > Pa
water_vp_sat(temperature = 20, over.ice = TRUE, method = "tetens") # C > Pa
water_vp_sat(temperature = 20, over.ice = TRUE, method = "wexler") # C > Pa
water_vp_sat(temperature = 20, over.ice = TRUE, method = "goff.gratch") # C > Pa
water_dp(water.vp = 1000) # Pa > C
water_dp(water.vp = 1000, method = "magnus") # Pa > C
water_dp(water.vp = 1000, method = "wexler") # Pa > C
water_dp(water.vp = 500, over.ice = TRUE) # Pa > C
water_dp(water.vp = 500, method = "wexler", over.ice = TRUE) # Pa > C
water_fp(water.vp = 300) # Pa > C
water_dp(water.vp = 300, over.ice = TRUE) # Pa > C
water_vp2RH(water.vp = 1500, temperature = 20) # Pa, C > RH %
water_vp2RH(water.vp = 1500, temperature = c(20, 30)) # Pa, C > RH %
water_vp2RH(water.vp = c(600, 1500), temperature = 20) # Pa, C > RH %
water_vp2mvc(water.vp = 1000, temperature = 20) # Pa > g m3
water_mvc2vp(water.mvc = 30, temperature = 40) # g m3 > Pa
water_dp(water.vp = water_mvc2vp(water.mvc = 10, temperature = 30)) # g m3 > C

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