What instrument measures air moisture content?

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Measuring how much moisture is in the air relies on how evaporation behaves in that air. A wet-bulb thermometer does this by having a wick soaked in water around its bulb. As air flows over the wick, water evaporates, pulling heat from the bulb and cooling it. The amount of cooling depends on the air’s moisture content: in dry air, evaporation is rapid and the wet-bulb temperature drops well below the dry-bulb; in humid air, evaporation slows and the two temperatures stay closer together. By comparing the wet-bulb reading to the dry-bulb reading (or using a psychrometric chart), you can determine relative humidity and other moisture measures. This evaporative cooling behavior makes the wet-bulb thermometer the best instrument for gauging air moisture content. The other devices measure different things—barometric pressure, wind speed, or humidity directly in some cases—but do not rely on evaporative cooling to reveal moisture content.