Defining relative humidity in terms of water activity: III. Relations to dew-point and frost-point temperatures

Abstract

Relative humidity (RH) is a fundamental quantity used in many fields of engineering and science, in particular also in meteorology and climate research. Relative fugacity (RF) and, equivalently, relative activity of water vapour in humid air have recently been proposed as a physically well-founded, unambiguous common metrological reference quantity for several conventional but mutually inconsistent definitions of RH. The RF definition is valid also beyond perfect-gas conditions and above boiling and sublimation temperatures. While differences between RH and RF mostly remain within uncertainties of typical present-day RH measurements, such systematic discrepancies are expected to be of substantial climatological relevance. Consequently, interdisciplinary harmonisation of RH definitions is overdue within the SI framework. Dew-point and frost-point temperatures are preferred measurands in humidity metrology using, for example, chilled-mirror hygrometers. Here, relations are presented for estimating RF from those temperatures, based on equations of state of the 2011 IUGG standard TEOS-10, the “International Thermodynamic Equation of Seawater – 2010”. Recommendations are given for numerically computing RF using the open-source TEOS-10 SIA Library . The asymptotic limiting laws of RF for nearly saturated humid air exhibit the familiar form of Clausius-Clapeyron-like equations, despite refraining from ideal-gas assumptions. Under various practical conditions, these simple equations may cover even the full humidity range with only minor residuals compared to the full numerical TEOS-10 solution for RF.

Irreversible Thermodynamics of Seawater Evaporation

“…Functions for numerous thermodynamic properties are implemented as an open-source code in the SIA ("Sea-Ice-Air") and the GSW ("Gibbs Seawater") libraries (Wright et al, 2010) [50], also available from in TEOS-10 web page. In an appendix of Feistel et al (2022) [51], an additional explicit TEOS-10 code is provided for the calculation of relative fugacity, a real gas alternative to conventional relative humidity, as defined mathematically by Feistel and Lovell-Smith (2017) [52]. Simplified TEOS-10 equations for application at the sea surface are reported in an appendix of Feistel and Hellmuth (2023) [18].…”

TEOS-10 Equations for the Lifted Condensation Level (LCL) and Climatic Feedback of Marine Clouds

“…In ideal-gas approximation, RF is identical with conventional RH in terms of water-vapour partial pressures. For air close to saturation, such as the marine surface layer with about 80 %rh, the Clausius-Clapeyron formula is an excellent approximation of TEOS-10 values for RF, even if refraining from the perfect-gas assumption (Feistel et al 2022). RF appears naturally in expressions for the Onsager driving force of non-equilibrium fluxes, such as evaporation from the ocean surface Hellmuth 2023, 2024).…”

“…RF appears naturally in expressions for the Onsager driving force of non-equilibrium fluxes, such as evaporation from the ocean surface Hellmuth 2023, 2024). For the numerical computation of RF, a source code extension to the SIA library is available from Feistel et al (2022).…”

Long-Term Forecasting of Air Pollution Particulate Matter (PM2.5) and Analysis of Influencing Factors

“…Low-pressure systems often create stagnant conditions, which lead to the accumulation of pollutants, including PM2.5, in the lower atmosphere. Dew point [21][22][23] represents the temperature at which air becomes saturated, causing water vapor condensation. Higher dew point values indicate increased moisture in the air, which can impact particle growth dynamics and increase the likelihood of particle hygroscopic growth, potentially leading to higher PM2.5 concentrations.…”