The general name for an instrument that measures the evaporation rate of water into the atmosphere. Four main classes of atmometers may be distinguished: 1) large evaporation tanks sunk into the ground or floating in water; 2) small evaporation pans; 3) porous porcelain bodies; and 4) porous paper wick devices. The evaporation from a surface depends greatly upon the nature of the surface and the exposure of the surface to the atmosphere. Measured evaporation rates should be compared only between identical instruments. See clay atmometer, evaporation pan, evapotranspirometer, Livingstone sphere, Piché evaporimeter, radio atmometer.