Buck Equation: P = 0.61121 × e^((18.678 − T/234.5) × T / (257.14 + T))
The vapour pressure of water is critical in meteorology, HVAC engineering, and chemistry. The Buck equation (1981) gives excellent accuracy from −80 to +50°C. Key values: at 0°C = 0.611 kPa (4.585 mmHg), 20°C = 2.338 kPa, 25°C = 3.169 kPa, 37°C = 6.275 kPa, 100°C = 101.325 kPa (760 mmHg, defining the normal boiling point). Relative humidity = (actual VP / saturation VP) × 100%. When collecting gases over water, P_gas = P_total − P_water(T). The Clausius-Clapeyron equation relates VP to enthalpy of vaporization: d(lnP)/d(1/T) = −ΔHvap/R. Water's high ΔHvap (40.7 kJ/mol) means it takes substantial energy to evaporate — this is why sweating cools the body and why steam burns are severe.