Ruivo, CelestinoCosta, Vítor A. F.Domínguez-Muñoz, Fernando2026-03-172026-03-172026-01-152374-4731http://hdl.handle.net/10400.1/28443The state of moist air significantly impacts human comfort, human health, preservation of goods and food, and many processes. At a given pressure, it is usually defined by one pair of independent variables, one invariably the dry bulb temperature, and the other the humidity ratio, the relative humidity, or the specific enthalpy. Relative humidity is a widely referred to and used concept. However, there is no consensus on its definition. The different definitions in literature are not clear enough, and are prone to misleading and questionable numerical values. This is especially relevant for high temperatures when using psychrometric charts, online calculators, simulation tools, software of measuring systems, and home-made routines or numerical tools. This work aims to highlight how and why the concept of relative humidity needs to be clearly and uniquely defined, and why care is needed concerning its applicability conditions. Additionally to the given explanations, mathematical expressions are used to do that more clearly and accurately. The main key issues raised in this paper are related to the consideration, and use of relative humidity when moist air temperature exceeds the water saturation temperature corresponding to the prevailing total pressure.engThermodynamic propertiesPsychrometric chartsMoist airWaterjournal article10.1080/23744731.2025.26073092374-474X