Thermodynamic Properties of Seawater and Humid Air

The thermodynamic properties of seawater and humid air encompass the relationships between temperature, pressure, salinity, humidity and phase equilibria that govern oceanic and atmospheric processes. In seawater, key variables include density, salinity and enthalpy, all of which are determined by an equation of state that accounts for the dissolved constituents. The international Thermodynamic Equation of Seawater 2010 provides a unified framework for calculating these properties across a wide range of conditions, enabling accurate computation of buoyancy, circulation and heat content. In humid air, the interaction with evaporating seawater introduces water vapour into the marine troposphere, altering heat capacity, enthalpy and buoyancy profiles. The concept of relative humidity and its more rigorous counterpart, relative fugacity, is central to describing phase transitions between liquid and vapour. Together, these properties underpin the exchange of heat, mass and momentum at the air–sea interface, influencing weather systems, climate feedbacks and the design of oceanographic instruments. Advances in computational methods and standardised potential functions have enhanced the precision of coupled atmosphere–ocean models, while improved metrological practices ensure traceability and reduced uncertainty in in situ measurements. The global importance of these developments spans climate prediction, marine engineering, sensor calibration and the sustainable management of marine resources.

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Thermodynamic Properties of Seawater and Humid Air publication trend

The graph below shows the total number of articles in thermodynamic properties of seawater and humid air across all publications each year (not limited to Nature Index journals).

Technical terms

Absolute Salinity: The true mass fraction of dissolved salts in seawater, reflecting spatial composition anomalies.

Practical Salinity: A conductivity‐based measure of salinity calibrated to a reference seawater composition.

Thermodynamic Equation of Seawater (TEOS-10): The international standard set of equations and potential functions for seawater and humid air properties.

Relative Humidity: The ratio of actual water‐vapour pressure to saturation vapour pressure at a given temperature.

Relative Fugacity: A rigorous alternative to relative humidity that accounts for non-ideal gas behaviour of water vapour.

Thermodynamic Potential: A state function (such as Gibbs or Helmholtz energy) from which all equilibrium properties can be derived.

Potential Enthalpy: The enthalpy of a fluid parcel referenced to a standard pressure, used in heat‐content calculations.

Conservative Temperature: A temperature variable proportional to potential enthalpy, ensuring conservation under adiabatic processes.

References

1. A Global Seawater Density Distribution Model Using a Convolutional Neural Network. Sensors (2024).
2. Essential Ocean Variables for Marine Environment Monitoring: Metrological Case Studies. Journal of Marine Science and Engineering (2023).
3. Mutually consistent thermodynamic potentials for fluid water, ice and seawater: a new standard for oceanography. Ocean Science (2008).
4. Numerical implementation and oceanographic application of the thermodynamic potentials of liquid water, water vapour, ice, seawater and humid air – Part 1: Background and equations. Ocean Science (2010).
5. Thermodynamic properties of sea air. Ocean Science (2010).
6. Absolute Salinity, ''Density Salinity'' and the Reference-Composition Salinity Scale: present and future use in the seawater standard TEOS-10. Ocean Science (2011).
7. The interpretation of temperature and salinity variables in numerical ocean model output and the calculation of heat fluxes and heat content. Geoscientific Model Development (2021).