Thevapor pressure of water is the pressure exerted by molecules of water vapor in gaseous form (whether pure or in a mixture with other gases such as air). Thesaturation vapor pressure is the pressure at whichwater vapor isin thermodynamic equilibrium with its condensed state. At pressures higher than saturation vapor pressure,water willcondense, while at lower pressures it willevaporate orsublimate. The saturation vapor pressure of water increases with increasingtemperature and can be determined with theClausius–Clapeyron relation. Theboiling point of water is the temperature at which the saturated vapor pressure equals the ambient pressure. Watersupercooled 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 (saturation) vapor pressure of water are commonly used inmeteorology. The temperature-vapor pressure relation inversely describes the relation between theboiling point of water and the pressure. This is relevant to bothpressure cooking and cooking at high altitudes. An understanding of vapor pressure is also relevant in explaining high altitudebreathing andcavitation.

There are many published approximations for calculating saturated vapor pressure over water and over ice. Some of these are (in approximate order of increasing accuracy):

Name	Formula	Description
"Eq. 1" (August equation)	${\displaystyle P=\exp \left(20.386-\frac{5132}{T}\right)}$	P is the vapour pressure inmmHg andT is the temperature inkelvins. Constants are unattributed.This is of the form that would be derived from theClausius-Clapeyron relation
TheAntoine equation	${\displaystyle {\log }_{10}P=A-\frac{B}{C+T}}$	T is indegrees Celsius (°C) and the vapour pressure P is inmmHg. The (unattributed) constants are given as A B C Tmin, °C Tmax, °C 8.07131 1730.63 233.426 1 99 8.14019 1810.94 244.485 100 374
August-Roche-Magnus (or Magnus-Tetens or Magnus) equation	${\displaystyle P=0.61094\exp \left(\frac{17.625T}{T+243.04}\right)}$	Temperature T is in °C and vapour pressure P is inkilopascals (kPa). The coefficients given here correspond to equation 21 in Alduchov and Eskridge (1996).[2] See alsodiscussion of Clausius-Clapeyron approximations used in meteorology and climatology.
Tetens equation	${\displaystyle P=0.61078\exp \left(\frac{17.27T}{T+237.3}\right)}$	T is in °C and  P is in kPa
TheBuck equation.	${\displaystyle P=0.61121\exp \left(\left(18.678-\frac{T}{234.5}\right)\left(\frac{T}{257.14+T}\right)\right)}$	T is in °C andP is in kPa.
TheGoff-Gratch (1946) equation.[3]	(See article; too long)

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 (2005):

T (°C)	P (Lide Table)	P (Eq 1)	P (Antoine)	P (Magnus)	P (Tetens)	P (Buck)	P (Goff-Gratch)
0	0.6113	0.6593 (+7.85%)	0.6056 (-0.93%)	0.6109 (-0.06%)	0.6108 (-0.09%)	0.6112 (-0.01%)	0.6089 (-0.40%)
20	2.3388	2.3755 (+1.57%)	2.3296 (-0.39%)	2.3334 (-0.23%)	2.3382 (-0.03%)	2.3383 (-0.02%)	2.3355 (-0.14%)
35	5.6267	5.5696 (-1.01%)	5.6090 (-0.31%)	5.6176 (-0.16%)	5.6225 (-0.07%)	5.6268 (+0.00%)	5.6221 (-0.08%)
50	12.344	12.065 (-2.26%)	12.306 (-0.31%)	12.361 (+0.13%)	12.336 (-0.06%)	12.349 (+0.04%)	12.338 (-0.05%)
75	38.563	37.738 (-2.14%)	38.463 (-0.26%)	39.000 (+1.13%)	38.646 (+0.21%)	38.595 (+0.08%)	38.555 (-0.02%)
100	101.32	101.31 (-0.01%)	101.34 (+0.02%)	104.077 (+2.72%)	102.21 (+0.88%)	101.31 (-0.01%)	101.32 (0.00%)

A more detailed discussion of accuracy and considerations of the inaccuracy in temperature measurements is presented in Alduchov and Eskridge (1996). 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 (e.g., in meteorology). As expected,^{[clarification needed]}Buck's equation forT > 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 complexGoff-Gratch equation over the range needed for practical meteorology.

For serious computation, Lowe (1977)^[4] developed two pairs of equations for temperatures above and below freezing, with different levels of accuracy. They are very accurate (compared toClausius-Clapeyron andGoff-Gratch) but use nested polynomials for efficient computation. However, there are more recent reviews of possibly superior formulations, notably Wexler (1976, 1977),^[5][6] reported by Flatau et al. (1992).^[7]

Examples of modern use of these formulae can additionally be found in NASA's GISS Model-E and Seinfeld and Pandis (2006). The former is an extremely simple Antoine equation, while the latter is a polynomial.^[8]

In 2018 a new physics-inspired approximation formula was devised and tested by Huang^[9] who also reviews other recent attempts.

• Dew point
• Gas laws
• Lee–Kesler method
• Molar mass

• Vömel, Holger (2016)."Saturation vapor pressure formulations". Boulder CO: Earth Observing Laboratory, National Center for Atmospheric Research. Archived fromthe original on June 23, 2017.
• "Vapor Pressure Calculator". National Weather Service, National Oceanic and Atmospheric Administration.

• Thermodynamic properties
• Atmospheric thermodynamics

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• Short description is different from Wikidata
• Wikipedia articles needing clarification from August 2023