Defining relative humidity in terms of water activity. Part 1: definition

Abstract

Relative humidity (RH) is a quantity widely used in various fields such as metrology, meteorology, climatology or engineering. However, RH is neither uniformly defined, nor do some definitions properly account for deviations from ideal-gas properties, nor is the application range of interest fully covered. In this paper, a new full-range definition of RH is proposed that is based on the thermodynamics of activities in order to include deviations from ideal-gas behaviour. Below the critical point of pure water, at pressures p < 22.064 MPa and temperatures T < 647.096 K, RH is rigorously defined as the relative activity (or relative fugacity) of water in humid air. For this purpose, reference states of the relative activity are specified appropriately. Asymptotically, the ideal-gas limit of the new definition is consistent with de-facto standard RH definitions published previously and recommended internationally. Virial approximations are reported for estimating small corrections to the ideal-gas equations.

Thermodynamic properties of seawater, ice and humid air: TEOS-10, before and beyond

“…For these physical reasons, TEOS-10 suggests a replacement of the usual definition of relative humidity in terms of partial pressures by a new definition in terms of fugacities (IOC et al, 2010, Sect. 3.40 therein;Feistel et al, 2010bFeistel et al, , 2015Feistel et al, , 2016Feistel, 2012;Feistel and Lovell-Smith, 2017). In fact, the small real-gas effects of the atmosphere are of similar magnitude as the expected changes in relative humidity associated with greenhouse effects (Hellmuth et al, 2018).…”

“…Beginning in 2006, TEOS-10 was developed in close cooperation between the SCOR/IAPSO Working Group 127 on Thermodynamics and Equation of State of Seawater (WG127) and the International Association for the Properties of Water and Steam (IAPWS). TEOS-10 is officially defined by its manual (IOC et al, 2010) and is supported by open-source software libraries (available from the internet at the TEOS-10 home page, www.teos-10.org, last access: 7 June 2018), several open-access IAPWS documents (IAPWS AN6-16, 2016) and various supporting scientific articles, such as the definition of the new Reference-Composition Salinity Scale and a new thermodynamic definition of relative humidity (Feistel and Lovell-Smith, 2017). A collection of background articles on TEOS-10 was published as a Special Issue of Ocean Science (Feistel et al, 2008b).…”

Accuracy of Approximations to the Poynting Correction for Ice and Liquid Water

“…Table 1 gives the coefficients Ai, Bi, Ci, and Di for Eqs. (5) and (6). Equations (5) and (6) reproduce the values from Refs.…”

“…It has recently been proposed [5,6] to make use of fugacity-based measures for the traditional concept of relative humidity, which would provide more impetus for simple and accurate ways to calculate the fugacity of liquid water and ice.…”

Reply to the short comments of Hannes Schmidt of 04 Apr 2018

“…Beginning in 2006, TEOS-10 was developed in close cooperation between the SCOR/IAPSO Working Group 127 on Thermodynamics and Equation of State of Seawater (WG127) and the International Association for the Properties of Water and Steam (IAPWS). TEOS-10 is officially defined by its manual (IOC et al, 2010) and is supported by open-source software libraries (available from the internet at the TEOS-10 home page, www.teos-10.org, last access: 7 June 2018), several open-access IAPWS documents (IAPWS AN6-16, 2016) and various supporting scientific articles, such as the definition of the new Reference-Composition Salinity Scale and a new thermodynamic definition of relative humidity (Feistel and Lovell-Smith, 2017). A collection of background articles on TEOS-10 was published as a Special Issue of Ocean Science (Feistel et al, 2008b).…”