Instruments that measure humidity are called hygrometers. Don’t confuse the word with “hydrometers”, which are used to measure specific gravity of fluids such as battery acid.
Dew-point hygrometers, also known as chilled-mirror hygrometers, reflect a beam of light off a tiny metal mirror. When the mirror is cooled to the dew-point temperature, dew forms on the mirror and the light beam scatters instead of reflecting into a detector. Electronics in the instrument cool or heat the mirror to maintain the surface precisely at the dew-point temperature, which is provided as an output. These are accurate instruments with relatively slow response. For cold temperatures and low humidities, frost-point hygrometers are used instead.
Hair hygrometers use organic fibers such as long hairs, anchored at one end and attached at the other end by amplifying levers to a dial that reads out relative humidity. As the RH increases, the hairs get longer, causing the dial to turn. These are inaccurate, but are inexpensive and are the most common hygrometers for home use. Any material that changes dimensions when absorbing water molecules can be used in hygrometers. One example is a bi-material coil that rotates as humidity changes.
Psychrometers are instruments with two liquid-in-glass thermometers attached to a board or frame. The bulb of one thermometer is surrounded by a sleeve or wick of cloth that is saturated with distilled water, while the other bulb remains dry. After both thermometers are actively ventilated [by whirling the instrument through the air on a handheld axel (sling psychrometer), or by using a spring or electrically driven fan to blow air past the thermometers (aspirated psychrometer)], the two thermometers are read to give the wet and drybulb temperatures. The wet-bulb is cooler than the dry, because of the latent heat absorbed when water evaporates. This thermodynamic information can be used with psychrometric tables or charts (Figs. 4.4 & 4.5) to determine the humidity. These instruments are extremely slow response, but relatively simple. Modern psychrometers replace the liquid-in-glass thermometers with electronic thermometers such as thermistors.
In old radiosondes (balloon-borne weather instruments), the electrical resistance across a carboncoated glass slide was measured. In more humid air, this carbon-film hygrometer becomes more resistive. Modern radiosondes often measure the capacitance across a very thin dielectric plastic that is coated on both sides with a porous metallic grid. Both approaches are small and light enough to be carried aloft, but both sensors can be easily contaminated by chemical vapors that change their electrical properties.
Microwave refractometers draw air into a small chamber filled with microwaves. The refraction (bending) of these microwave beams depends on humidity (see the radar section of the Satellites & Radar chapter), and can be measured. These are very fast-response sensors.
Spectral absorption hygrometers, also known as optical hygrometers, transmit frequencies of electromagnetic radiation that are strongly absorbed by water vapor. By passing the beam of radiation across a short path of air to a detector, the amount of attenuation can be measured to allow calculation of the absolute humidity. One such instrument, the Lyman-alpha hygrometer, uses ultraviolet light of wavelength 0.121567 µm, corresponding to an absorption/emission line of hydrogen. Another, the krypton hygrometer, uses emissions at 0.12358 µm, generated by a glow tube filled with the noble gas krypton. Other instruments use absorption of infrared light (infrared hygrometers). These are all fast-response instruments. See the Satellites & Radar chapter for absorption spectra across the atmosphere.
Some lidars (laser radars) have been developed to transmit two neighboring wavelengths of electromagnetic radiation, one of which is affected by water vapor and the other which is not. Such differential absorption lidars (DIAL) can remotely measure humidity along vertical or slant paths, and can scan the atmosphere to measure the humidity in a volume or in a plane.
Weather radars and other microwave profilers can be used to measure profiles of humidity in the atmosphere, because the speed and/or polarity of microwaves through air depends on humidity.
Some sensors measure path-averaged humidity. One example is the water-vapor channel on weather satellites, which measures infrared emissions from water vapor in the air. As discussed in the Satellites & Radar chapter, such emissions come from a layer of air several kilometers thick in the top third of the troposphere. These instruments have the advantage of remotely sampling the atmosphere at locations that are difficult to reach otherwise, such as over the oceans. A disadvantage is that they have difficulty seeing through clouds.
Transmissions from Global Positioning System (GPS) satellites are slightly delayed or refracted by humidity along the path of the beam through the atmosphere. However, data from many such crossing beams from the constellation of GPS satellites can be computationally inverted to yield vertical profiles of humidity, similar to the medical X-ray tomography methods used for brain scans.