What is a hygrometer?

   HYGROMETER is an instrument used to measure the amount of water vapor in the air. Meteorologists use hygrometers to determine the relative humidity of the air. The most common types of hy­grometers are the psychrometer and the hair hygrometer.

   The Psychrometer consists of two thermometers mounted on the same frame. The frame is designed so it can be rotated. The bulb of one thermometer is covered with a tight-fitting muslin sack, and wetted with water. This thermometer is known as the wet-bulb thermometer. The other is the dry-bulb thermometer. The psychrometer is rotated at a speed of about 9 miles an hour to provide ventilation for the bulbs. The dry bulb indicates the temperature of the air. The wet bulb helps determine the relative humidity.
   When the psychrometer whirls through the air, water from the muslin evaporates. The evaporating water cools the wet bulb. The amount of cooling that occurs depends on the relative humidity. The lower the humid­ity, the faster the water in the muslin will evaporate, and the more the bulb will cool. A high amount of humidity will cause less evaporation, slowing the cooling process.
   In air that has less than 100 percent relative humid­ity, the wet bulb will record a lower temperature than the dry bulb. This difference in temperature is known as wet-bulb depression. A convenient chart shows the wet-bulb depressions at various temperatures. By using the chart, the observer can determine the relative humidity of the air.
   The Assman psychrometer is gradually becoming the accepted type of hygrometer, because of its high accuracy. It has a built-in fan that draws the proper amount of air through two metal tubes that hold the thermom­eters.

   The Hair Hygrometer uses a bundle of human hairs to detect relative humidity. The length of the hair increases as it absorbs moisture from the air. One end of the bundle is anchored. The other end is attached to a lever that moves a pointer on a scale. As the humidity lengthens or shortens the hair, the lever moves the pointer across the scale, indicating the amount of rela­tive humidity.
   A new type of hygrometer gives a direct reading of the dew-point temperature, or the temperature at which the air has too percent relative humidity. This instrument has a polished surface that is alternately cooled and heated until dew is formed and evaporated. A device on the surface indicates the temperature at which dew is formed.