An evaporation pan is a standardized open container used to measure the rate of water evaporation at a specific location. The Class A pan is the most widely used design worldwide.
An evaporation pan is a meteorological instrument used to estimate evaporation rates from open water surfaces by measuring the loss of water from a standardized container exposed to natural atmospheric conditions. The U.S. National Weather Service Class A pan is the international standard: a cylindrical galvanized steel pan 1.21 meters (4 feet) in diameter and 25.4 centimeters (10 inches) deep, mounted on a wooden platform 15 centimeters above the ground. Water levels are measured daily using a hook gauge or stilling well, and pan evaporation is calculated as the difference between measured water level and the previous day's level, adjusted for any rainfall. Because the pan is smaller and more exposed than natural water bodies, pan evaporation rates are higher than actual lake evaporation. A pan coefficient (typically 0.7 for the Class A pan) is applied to convert pan evaporation to estimated lake evaporation. Pan evaporation data are used in water balance calculations, irrigation scheduling, reservoir management, and reference evapotranspiration estimation. Global pan evaporation networks have revealed a phenomenon called the evaporation paradox, where pan evaporation rates have decreased in many regions despite rising temperatures, attributed to changes in solar radiation and wind speed.