Vapour pressure of water


T, °CT, °FP, kPaP, TorrP, atm
0320.61134.58510.0060
5410.87266.54500.0086
10501.22819.21150.0121
15591.705612.79310.0168
20682.338817.54240.0231
25773.169023.76950.0313
30864.245531.84390.0419
35955.626742.20370.0555
401047.381455.36510.0728
451139.589871.92940.0946
5012212.344092.58760.1218
5513115.7520118.14970.1555
6014019.9320149.50230.1967
6514925.0220187.68040.2469
7015831.1760233.83920.3077
7516738.5630289.24630.3806
8017647.3730355.32670.4675
8518557.8150433.64820.5706
9019470.1170525.92080.6920
9520384.5290634.01960.8342
100212101.3200759.96251.0000

The vapor pressure of water is the pressure exerted by molecules of water vapor in gaseous form. The saturation vapor pressure is the pressure at which water vapor is in thermodynamic equilibrium with its condensed state. At pressures higher than saturation vapor pressure, water will condense, while at lower pressures it will evaporate or sublimate. The saturation vapor pressure of water increases with increasing temperature and can be determined with the Clausius–Clapeyron relation. The boiling point of water is the temperature at which the saturated vapor pressure equals the ambient pressure. Water supercooled below its normal freezing point has a higher vapor pressure than that of ice at the same temperature and is, thus, unstable.
Calculations of the vapor pressure of water are commonly used in meteorology. The temperature-vapor pressure relation inversely describes the relation between the boiling point of water and the pressure. This is relevant to both pressure cooking and cooking at high altitudes. An understanding of vapor pressure is also relevant in explaining high altitude breathing and cavitation.

Approximation formulas

There are many published approximations for calculating saturated vapor pressure over water and over ice. Some of these are :
NameFormulaDescription--
"Eq. 1" is the vapour pressure in mmHg and is the temperature in kelvins.
Constants are unattributed.
This is of the form that would be derived from the Clausius-Clapeyron relation		--
The Antoine equation is in degrees Celsius and the vapour pressure is in mmHg. The constants are given as

Accuracy of different formulations

Here is a comparison of the accuracies of these different explicit formulations, showing saturation vapor pressures for liquid water in kPa, calculated at six temperatures with their percentage error from the table values of Lide :
A more detailed discussion of accuracy and considerations of the inaccuracy in temperature measurements is presented in Alduchov and Eskridge. The analysis here shows the simple unattributed formula and the Antoine equation are reasonably accurate at 100 °C, but quite poor for lower temperatures above freezing. Tetens is much more accurate over the range from 0 to 50 °C and very competitive at 75 °C, but Antoine's is superior at 75 °C and above. The unattributed formula must have zero error at around 26 °C, but is of very poor accuracy outside a narrow range. Tetens' equations are generally much more accurate and arguably more straightforward for use at everyday temperatures. As expected, Buck's equation for > 0 °C is significantly more accurate than Tetens, and its superiority increases markedly above 50 °C, though it is more complicated to use. The Buck equation is even superior to the more complex Goff-Gratch equation over the range needed for practical meteorology.

Numerical approximations

For serious computation, Lowe developed two pairs of equations for temperatures above and below freezing, with different levels of accuracy. They are very accurate but use nested polynomials for efficient computation. However, there are more recent reviews of possibly superior formulations, notably Wexler, reported by Flatau et al..
Examples of modern use of these formulae can additionally be found in NASA's GISS Model-E and Seinfeld and Pandis. The former is an extremely simple Antoine equation, while the latter is a polynomial.
In 2018 a new physics-inspired approximation formula was devised and tested by Huang who also reviews other recent attempts.

Graphical pressure dependency on temperature