A hygrometer measures the relative humidity in the air. Relative humidity is expressed as a percentage; it compares the amount of moisture, or water vapor, in the air to the maximum amount of moisture the air can potentially hold. Relative humidity is measured on a scale of zero to 100; the higher the number, the more moisture in the air.
Meteorologists routinely report relative humidity as part of the daily weather report, especially during the warmer parts of the year when it tends to reach levels close to the maximum 100 percent. An hydrometer is an essential part of their forecasting. One use of the hydrometer is the combination of its reading and that of a thermometer into a figure known as the heat index. This calculation is meant to explain how hot it feels in the summer. Whenever the relative humidity gets above 40 percent, the heat index will be higher than the actual temperature. An example is a day with a temperature of 90 degrees and a relative humidity of 70 percent. The heat index will be 105 according to NOAA's National Weather Service chart. A heat index of 105 and above is considered dangerous to human health.
A home hygrometer measures the inside relative humidity. Determine a comfortable level and then use the hygrometer as a monitor. When there is too much humidity, a dehumidifier can be turned on to remove moisture from the air. Turn it off if the air becomes too dry. Too much humidity in a home leads to mold and mildew growth and health issues. If the air is constantly dry a humidifier adds moisture back into the home, with the hygrometer monitoring the amount of moisture added.
Hygrometers have several commercial uses, including monitoring humidity in storage facilities for old books, food, furniture, musical instruments and other items that can be damaged by moisture. Saunas, whether commercial or residential, also employ a hygrometer in tandem with a thermometer to monitor the air. Another use involves a cigar humidor. Humidity greatly affects the quality of the tobacco in cigars that are stored over a period of time. Proper temperature and humidity is essential, requiring a hygrometer.
Museums contain valuable works of art, artifacts, papers and other rare and ancient items that are very sensitive to temperature and humidity. Protecting them from decay and destruction requires a continual effort to ensure the indoor conditions are not harmful. An hygrometer is a vital part of that protection. High relative humidity is harmful and must be avoided. Hygrometers that keep a log of relative humidity levels provide valuable information. Hand-held units provide instant readings in all parts of the museum.