Gary Archibald, Weathercaster

The fourth installment of the weather devices/instruments blog series answers the question: What weather device measures the moisture content in the air (humidity)? The answer is: a hygrometer.

Hygrometers are instruments used for measuring humidity. A simple form of a hygrometer is specifically known as a "psychrometer" and consists of two thermometers, one of which includes a dry bulb and the other of which includes a bulb that is kept wet to measure wet-bulb temperature.

Evaporation from the wet bulb lowers the temperature, so that the wet-bulb thermometer usually shows a lower temperature than that of the dry-bulb thermometer, which measures dry-bulb temperature. When the air temperature is below freezing, however, the wet bulb is covered with a thin coating of ice and yet may be warmer than the dry bulb.

Relative Humidity is computed from the ambient temperature as shown by the dry-bulb thermometer and the difference in temperatures as shown by the wet-bulb and dry-bulb thermometers. Relative humidity can also be determined by locating the intersection of the wet- and dry-bulb temperatures on a psychrometric chart. One device that uses the wet/dry bulb method is the sling psychrometer, where the thermometers are attached to a handle or length of rope and spun around in the air for a few minutes.

A sling psychrometer

Other types of hygrometers are also commonly used to determine the ambient humidity. Such devices frequently use a human or animal hair under tension. The traditional folk art device known as a "weather house" works on this principle. In order to see changes that occur over time, several hygrometers record the value of humidity on a piece of graduated paper so that the values can be read off the chart.

Hydrometers are valuable tools especially during active rainy seasons particular to the Cetranl and southern Plains of the U.S. (during the early Spring to late Summer).

Next up: An instrument (peculiar in nature) used to measure the amount of sunlight falling on a specific location. Any guesses? Stay tuned!

GA