Hygrometers are instuments used to measure humidity.
Meteorologists use numerous parameters to study weather and climate, one of which is humidity. Humidity describes the amount of water vapor in the air and is measured by means of instruments called hygrometers. Common types of hygrometers include the psychrometer, the hair hygrometer, the dew point hygrometer, the electrical hygrometer, dew cells and the infrared hygrometer. Relative humidity and dew point are terms commonly used to express humidity.
Psychrometer
The sling psychrometer is one of the most common tools used for measuring humidity. The device holds two identical mercury thermometers, one of which has a moistened cloth wick around its bulb. When the sling is twirled above the head at a high rate of speed, moisture evaporates from the wick and cools the wet-bulb thermometer. Relative humidity, or the amount of water vapor in the air compared to the total amount of water vapor the air can hold, is determined using wet-bulb and dry-bulb temperatures and psychometric charts. Dryer air yields lower wet-bulb temperatures.The Assmann ventilated psychrometer is an adaptation of the sling psychrometer. It uses a fan rather than twirling to promote wet-bulb evaporation and cooling.
Hair Hygrometer
Hair is sensitive to changes in relative humidity. It stretches when humidity rises and contracts when humidity decreases. The hair hygrometer consists of human or horse hair strands stretched between a stationary surface and a mechanical lever system. An ink pen attached to the lever system moves up and down as the hair lengthens or shortens and records the changes on a rotating cylinder, or hygrograph. Hair hygrometers are less accurate than sling hygrometers, require frequent calibration and produce large errors when humidity is either very high or very low.
Dew Point Hygrometer
Dew point is the temperature at which water vapor within the air begins to condense and form dew. A dew point hygrometer measures the temperature at which condensation forms on a cold, mirrored surface. The mirror is cooled and the temperature tracked until photoelectric cells detect dew or frost formation. The higher the dew point, the more moisture in the air. According to meteorologist Jeff Haby, dew points higher than 65 degrees Fahrenheit make it feel sticky and humid outside.
Other Hygrometers
Electrical hygrometers measure humidity by passing an electrical current through a carbon-coated plate. Resistance across the plate changes due to the absorption or release of water vapor that occurs when the humidity changes. Dew cells determine dew points. A lithium chloride solution is heated until it begins to evaporate. The temperature at which evaporation occurs is then used to calculate dew point. An infrared hygrometer measures infrared light absorption as it passes through the air. Absorption is greater when humidity is higher.
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