R: Water vapour pressure

water_vp_sat {photobiology} | R Documentation

## Water vapour pressure

### Description

Approximate water pressure in air as a function of temperature, and its inverse the calculation of dewpoint.

### Usage

    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_vp2mvc(water.vp, temperature)

    water_mvc2vp(water.mvc, temperature)

### Arguments

`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" and modified "magnus" 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 `method` used.
`water.vp` | numeric vector of water vapour pressure in air (Pa).
`water.mvc` | numeric vector of water vapour concnetration as mass per volume (g m-3).

### Details

This is an implementation of Tetens (1930) equation for the cases of equilibrium with a water and an ice surface and of the modified Magnus equations of Alduchov and Eskridge (1996, eqs. 21 and 23).

The equations are approximations, and in spite of the different names, 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 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.

### Value

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 m-3) for `water_vp2mvc`.

### Note

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.

### References

Tetens, O., 1930. Uber einige meteorologische Begriffe. Zeitschrift fur Geophysik, Vol. 6:297.

Alduchov, O. A., Eskridge, R. E., 1996. Improved Magnus Form Approximation of Saturation Vapor Pressure. Journal of Applied Meteorology, 35: 601-609 .

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)

### Examples

    water_vp_sat(20) # C -> Pa
    water_vp_sat(-10) # over water!!
    water_vp_sat(-10, over.ice = TRUE)
    water_vp_sat(20) / 100 # C -> mbar

    water_dp(1000) # Pa -> C

    water_vp2mvc(1000, 20) # Pa -> g m-3

    water_mvc2vp(30, 40) # g m-3 -> Pa

    water_dp(water_mvc2vp(10, 30)) # g m-3 -> C

[Package photobiology version 0.9.21 Index]