John Dalton first introduced a method for reliably measuring the dewpoint, accomplished by determining the highest temperature of water in a glass container that condenses water on its exterior (Dalton 1802b,a). This temperature was originally referred to as the “vapor point” or “point of condensation,” but the term “dewpoint” came into use shortly thereafter (Howard 1818; Dalton 1824). In the years since, many different analytic expressions have been proposed for the dewpoint in terms of temperature and relative humidity (e.g., Mitchell 1967; Wanielista et al. 1997; Sargent 1980; Urbank et al. 2001; Lawrence 2005), but only one has been derived theoretically and that one, as we will see, has errors that can exceed 1 K. This short note derives, from first principles, analytic expressions for the dewpoint and frost point that are accurate to within hundredths of a degree.
Given T and RHl (or RHs), the most accurate value of the dew (or frost) point is obtained by using an empirical expression for
To this end, we will use the approximations of Rankine (1866) and Kirchhoff (1858), which assume an ideal gas, constant heat capacities, and zero specific volume of condensates. Following Romps (2021, manuscript submitted to Quart. J. Roy. Meteor. Soc.), we may refer to this trio of assumptions as the Rankine–Kirchhoff (RK) approximations, which have been used to derive accurate, explicit, and analytic expressions for the equivalent potential temperature (Romps and Kuang 2010; Hauf and Höller 1987; Emanuel 1994), the quantity conserved by an adiabatically lifted parcel (Romps 2015), and the lifting condensation level (Romps 2017). Here, we aim to add to that list the dewpoint and frost point.
Before we calculate the errors in expressions for the dew and frost points, let us first estimate the uncertainties from modern laboratory measurements. We can estimate the empirical uncertainty in the dewpoint by taking the reported relative uncertainty in laboratory measurements of the vapor pressure
A similar expression may be obtained for Tf, replacing RHl with RHs and replacing the latent heat of evaporation with the approximated sum of the latent heats of evaporation and sublimation, Le0 + Lm0 = E0υ + E0s + Rυ. When calculating these expressions, the values used for E0υ, E0s, and Rυ are the same as those given above.
The errors in these equal-heat-capacity dewpoint and frost-point expressions can be calculated by subtracting from them the dewpoints and frost points obtained (by use of a root solver) from laboratory measurements of the saturation vapor pressure. For this purpose, we will use Eq. (10) of Murphy and Koop (2005) for

Errors in the (left) frost point and (right) dewpoint as obtained from the Rankine–Kirchhoff approximations (solid) or the approximation of equal heat capacities (dashed). Note that the ordinates differ between the left and right panels. Errors are calculated relative to laboratory measurements of vapor pressure summarized by the expressions of Murphy and Koop (2005) for liquid and Wagner et al. (2011) for ice. See the text for a comparison of these errors to the uncertainties from laboratory measurements.
Citation: Journal of the Atmospheric Sciences 78, 7; 10.1175/JAS-D-20-0301.1

Errors in the (left) frost point and (right) dewpoint as obtained from the Rankine–Kirchhoff approximations (solid) or the approximation of equal heat capacities (dashed). Note that the ordinates differ between the left and right panels. Errors are calculated relative to laboratory measurements of vapor pressure summarized by the expressions of Murphy and Koop (2005) for liquid and Wagner et al. (2011) for ice. See the text for a comparison of these errors to the uncertainties from laboratory measurements.
Citation: Journal of the Atmospheric Sciences 78, 7; 10.1175/JAS-D-20-0301.1
Errors in the (left) frost point and (right) dewpoint as obtained from the Rankine–Kirchhoff approximations (solid) or the approximation of equal heat capacities (dashed). Note that the ordinates differ between the left and right panels. Errors are calculated relative to laboratory measurements of vapor pressure summarized by the expressions of Murphy and Koop (2005) for liquid and Wagner et al. (2011) for ice. See the text for a comparison of these errors to the uncertainties from laboratory measurements.
Citation: Journal of the Atmospheric Sciences 78, 7; 10.1175/JAS-D-20-0301.1
The error in the equal-heat-capacity expression for the frost point is less than or equal to 0.05 K, which is the same order of magnitude as the uncertainty from laboratory measurements. For the dewpoint, however, the assumption of cpυ = cυl generates errors that far exceed the uncertainties from empirical data. For low relative humidities (less than or equal to about 0.2) or for high air temperatures (exceeding about 310 K, 37°C, or 100°F), the dewpoint error can approach and exceed 0.5°–1.0°C or 1°–2°F.
Repeating this calculation for the Rankine–Kirchhoff expressions in Eqs. (5) and (7) gives the solid curves in Fig. 1. We see that the errors in the RK dewpoint and RK frost point are less than 0.04 and 0.07 K, respectively. These errors are the same order of magnitude as the uncertainties from the laboratory measurements. With maximum errors measured in hundredths of a kelvin, the Rankine–Kirchhoff expressions for the dewpoint and frost point are sufficiently accurate for most, if not all, atmospheric applications.
To give a graphical summary of the Rankine–Kirchhoff dewpoint and frost-point depressions, Fig. 2 plots min[T − Td(T, RH), T − Tf(T, RH)] (i.e., the smaller of either the dewpoint depression or the frost-point depression) as a function of temperature and relative humidity (with respect to the saturation vapor pressure over liquid for T ≥ 273.16 K and over solid for T < 273.16 K). The kink in the curves at a temperature of 273.16 K marks the transition from relative humidity with respect to solid (below the kink) and with respect to liquid (above the kink). A second locus of kinks, marked by the dashed line, occurs where the dewpoint depression and frost-point depression are equal. Above the dashed line, dew forms before frost as the air is cooled, and vice versa below the dashed line, assuming readily available surfaces for both condensation and deposition. Code to evaluate Eqs. (5) and (7) are available on the author’s website in R, Python, FORTRAN, and MATLAB.

Minimum of the dewpoint depression and frost-point depression plotted as a function of temperature and relative humidity (with respect to the saturation vapor pressure over liquid for T ≥ 273.16 K and over solid for T < 273.16 K). The dashed line separates the regions where, as the air is cooled, dew forms first (red) and frost forms first (blue).
Citation: Journal of the Atmospheric Sciences 78, 7; 10.1175/JAS-D-20-0301.1

Minimum of the dewpoint depression and frost-point depression plotted as a function of temperature and relative humidity (with respect to the saturation vapor pressure over liquid for T ≥ 273.16 K and over solid for T < 273.16 K). The dashed line separates the regions where, as the air is cooled, dew forms first (red) and frost forms first (blue).
Citation: Journal of the Atmospheric Sciences 78, 7; 10.1175/JAS-D-20-0301.1
Minimum of the dewpoint depression and frost-point depression plotted as a function of temperature and relative humidity (with respect to the saturation vapor pressure over liquid for T ≥ 273.16 K and over solid for T < 273.16 K). The dashed line separates the regions where, as the air is cooled, dew forms first (red) and frost forms first (blue).
Citation: Journal of the Atmospheric Sciences 78, 7; 10.1175/JAS-D-20-0301.1
Acknowledgments
This work was supported by the U.S. Department of Energy’s (DOE) Atmospheric System Research (ASR), an Office of Science, Office of Biological and Environmental Research program; Lawrence Berkeley National Laboratory is operated for the DOE by the University of California under Contract DE-AC02-05CH11231. Code to calculate the Rankine–Kirchhoff dewpoint and frost point may be downloaded from https://romps.berkeley.edu/papers/pubs-2020-dewpoint.html.
REFERENCES
August, E. F., 1828: Ueber die Berechnung der Expansivkraft des Wasserdunstes. Ann. Phys., 89, 122–137, https://doi.org/10.1002/andp.18280890511.
Beltramino, G., L. Rosso, R. Cuccaro, S. Tabandeh, D. Smorgon, and V. Fernicola, 2020: Accurate vapor pressure measurements of supercooled water in the temperature range between 252 K and 273 K. J. Chem. Thermodyn., 141, 105944, https://doi.org/10.1016/j.jct.2019.105944.
Bosen, J. F., 1958: An approximation formula to compute relative humidity from dry bulb and dew point temperatures. Mon. Wea. Rev., 86, 486, https://doi.org/10.1175/1520-0493(1958)086<0486:AAFTCR>2.0.CO;2.
Callendar, H. L., 1911: Vaporization. The Encyclopaedia Britannica: A Dictionary of Arts, Sciences, Literature and General Information, 11th ed. Vol. 27, Cambridge University Press, 897–904.
Corless, R. M., G. H. Gonnet, D. E. G. Hare, D. J. Jeffrey, and D. E. Knuth, 1996: On the Lambert W function. Adv. Comput. Math., 5, 329–359, https://doi.org/10.1007/BF02124750.
Dalton, J., 1802a: Meteorological observations. Mem. Lit. Philos. Soc. Manchester, 5, 666–674.
Dalton, J., 1802b: On the constitution of mixed gases; on the force of steam or vapour from water and other liquids in different temperatures, both in a Torricellian vacuum and in air; on evaporation; and on the expansion of gases by heat. Mem. Lit. Philos. Soc. Manchester, 5, 535–602.
Dalton, J., 1824: Observations in meteorology, particularly with regard to the dew-point, or quantity of vapour in the atmosphere; made on the mountains in the north of England. Mem. Lit. Philos. Soc. Manchester, 4, 104–124.
Emanuel, K. A., 1994: Atmospheric Convection. Oxford University Press, 580 pp.
Hauf, T., and H. Höller, 1987: Entropy and potential temperature. J. Atmos. Sci., 44, 2887–2901, https://doi.org/10.1175/1520-0469(1987)044<2887:EAPT>2.0.CO;2.
Howard, L., 1818: The Climate of London, Deduced from Meteorological Observations, Made at Different Places in the Neighbourhood of the Metropolis. W. Phillips, 358 pp.
IAPWS, 2011: Revised release on the pressure along the melting and sublimation curves of ordinary water substance. IAPWS R14-08(2011), 7 pp., http://www.iapws.org/relguide/MeltSub.html.
IAPWS, 2015: Guideline on thermodynamic properties of supercooled water. IAPWS G12-15, 10 pp., http://www.iapws.org/relguide/Supercooled.html.
Kirchhoff, G., 1858: Ueber die Spannung des Dampfes von Mischungen aus Wasser und Schwefelsäure. Ann. Phys. Chem., 180, 612–621, https://doi.org/10.1002/andp.18581800806.
Lawrence, M. G., 2005: The relationship between relative humidity and the dewpoint temperature in moist air: A simple conversion and applications. Bull. Amer. Meteor. Soc., 86, 225–234, https://doi.org/10.1175/BAMS-86-2-225.
Magnus, G., 1844: Versuche über die Spannkräfte des Wasserdampfs. Ann. Phys. Chem., 137, 225–247, https://doi.org/10.1002/andp.18441370202.
Mitchell, L. V., 1967: Radiosonde dew-point accuracies 40°C to −40°C. U.S. Air Force Tech. Rep. 198, 27 pp., https://apps.dtic.mil/dtic/tr/fulltext/u2/659760.pdf.
Murphy, D. M., and T. Koop, 2005: Review of the vapour pressures of ice and supercooled water for atmospheric applications. Quart. J. Roy. Meteor. Soc., 131, 1539–1565, https://doi.org/10.1256/qj.04.94.
Rankine, W. J. M., 1866: On saturated vapours. Proc. Roy. Soc. Edinburgh, 31, 199–201.
Romps, D. M., 2015: MSE minus CAPE is the true conserved variable for an adiabatically lifted parcel. J. Atmos. Sci., 72, 3639–3646, https://doi.org/10.1175/JAS-D-15-0054.1.
Romps, D. M., 2017: Exact expression for the lifting condensation level. J. Atmos. Sci., 74, 3891–3900, https://doi.org/10.1175/JAS-D-17-0102.1.
Romps, D. M., and Z. Kuang, 2010: Do undiluted convective plumes exist in the upper tropical troposphere? J. Atmos. Sci., 67, 468–484, https://doi.org/10.1175/2009JAS3184.1.
Sargent, G. P., 1980: Computation of vapour pressure, dew-point and relative humidity from dry- and wet-bulb temperatures. Meteor. Mag., 109, 238–246.
Urbank, T. M., S. M. Kelly, T. O. King, and C. A. Archibald, 2001: Development and application of an integrated dew point and glass temperature sensor. SAE Tech. Paper Series 2001-01-0585, 13 pp.
Wagner, W., and A. Pruß, 2002: The IAPWS formulation 1995 for the thermodynamic properties of ordinary water substance for general and scientific use. J. Phys. Chem. Ref. Data, 31, 387–535, https://doi.org/10.1063/1.1461829.
Wagner, W., T. Riethmann, R. Feistel, and A. H. Harvey, 2011: New equations for the sublimation pressure and melting pressure of H2O ice Ih. J. Phys. Chem. Ref. Data, 40, 043103, https://doi.org/10.1063/1.3657937.
Wanielista, M., R. Kersten, and R. Eaglin, 1997: Hydrology: Water Quantity and Quality Control. 2nd ed. John Wiley and Sons, 567 pp.