evaporimeter [key], instrument that measures the rate of evaporation of water into the atmosphere, sometimes called an atmometer. Evaporimeters are of two types, those that measure the evaporation rate from a free water surface and those that measure it from a continuously wet porous surface. In the first type, the level of water in a tank or pan, often sunk into the ground so that the water surface is at ground level, is measured by a micrometer gauge. After accounting for increases due to rain and decreases due to deliberate draining, the day-to-day decrease in the water level can be attributed to evaporation. In one evaporimeter of the second type, the evaporation rate is computed according to the rate of weight loss of a wet pack of absorbent material. The Piché evaporimeter uses an inverted graduated cylinder of water with a filter-paper seal at the mouth. Evaporation takes place from the wet filter paper and thus depletes the water in the cylinder, so that the rate of evaporation can be read directly from the graduations marking the water level. The Livingston sphere, another evaporimeter of the second type, uses a wet ceramic sphere as the evaporating surface to simulate evaporation rates from vegetation. Because evaporation rates are so sensitive to the water supply, and the nature of the evaporating surface, data collected by evaporimeters often do not reflect true evaporation processes; hence, evaporimeters have limited use.
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