Water Vapour and Humidity
The measurement and thermodynamics of water vapour in air, the most variable and radiatively important atmospheric constituent.
Definition
Humidity is the amount of water vapour present in air, expressed through various measures, and its thermodynamics is governed by the equilibrium between vapour and condensed phases of water.
Scope
Covers measures of atmospheric moisture including vapour pressure, mixing ratio, specific humidity, relative humidity and dew point; the Clausius-Clapeyron relation governing saturation vapour pressure; the dependence of saturation on temperature; and the role of water vapour in latent-heat transport and as a greenhouse gas.
Core questions
- How is the moisture content of air quantified, and how do the measures relate?
- Why does the maximum water vapour air can hold rise steeply with temperature?
- How does water vapour transport energy and influence the radiation balance?
Key theories
- Clausius-Clapeyron relation
- A thermodynamic equation showing that saturation vapour pressure increases nearly exponentially with temperature, setting the moisture-holding capacity of air.
Mechanisms
At any temperature air can hold water vapour up to a saturation vapour pressure that rises steeply with temperature according to the Clausius-Clapeyron relation, roughly seven percent per degree Celsius. Relative humidity is the ratio of actual to saturation vapour pressure; the dew point is the temperature to which air must cool to reach saturation. Evaporation and condensation transfer large amounts of latent heat, coupling the water cycle to the atmospheric energy budget.
Clinical relevance
Humidity measures are essential to weather forecasting, the prediction of cloud and fog formation, human comfort and heat stress assessment, and understanding the water-vapour feedback in climate.
History
The thermodynamics of phase equilibrium underlying saturation vapour pressure was established by Clapeyron and Clausius in the nineteenth century, providing the quantitative basis for humidity that remains central to atmospheric science.
Key figures
- Rudolf Clausius
- Benoit Clapeyron
Related topics
Seminal works
- iribarne1981
Frequently asked questions
- What is the dew point?
- The dew point is the temperature to which air must be cooled, at constant pressure and moisture content, for it to become saturated and for condensation to begin.