Water and Its Transformations

Abstract

Abstract Because water vapor is a minor (≈1%) component it contributes little to the thermodynamic and transport properties of air. Unlike nitrogen and oxygen, water’s critical temperature (647 K) is above tropospheric temperatures, and hence, can change phase. Evaporation of liquid water and condensation of water vapor are independent processes; their difference, net evaporation, is readily measured. If evaporation equals condensation, the partial pressure of water vapor is called the equilibrium (saturation) vapor pressure. The partial pressure of water vapor in air is usually below saturation. Its value relative to what it would be at saturation is the relative humidity. The Clausius–Clapeyron equation predicts that evaporation of liquid water increases exponentially with absolute temperature. Water evaporates at all temperatures, not just the boiling point, Although the normal range at which pure liquid water exists (at sea level) is 0°C to 100°C, the actual range is about −40°C (subcooled) to 280°C (superheated). All phase changes, including boiling, require nucleation.