There are various methods of humidity measurement, most of them using a device called a hygrometer. The hygrometer can work in a variety of ways, depending on the type, and is the most accurate method for determining the humidity in the air. Some are connected to other devices called humistats, which are connected to humidifiers and dehumidifiers, and help control the level of humidity in the air.
One type of hygrometer uses hair, usually human, that is attached to levers inside the unit. When the humidity in the air increases, the hair stretches, and then contracts when it decreases. The levers are generally connected to a dial which displays the humidity reading.
Other methods of humidity measurement using a hygrometer include the psychrometer as well as electric hygrometers. Both methods are fairly accurate, although the electric version is more sophisticated in design. There are also chemical hygrometers that are similar in effectiveness.
The psychrometer, also known as the two-bulb hygrometer, works by using two thermometer bulbs. One bulb is dry and measures the temperature in the air. The other bulb is covered in a substance, usually wick or muslin, and then wetted down. After being wetted, the bulb is exposed to moving air, either through a fan or by slinging the psychrometer through the air. As the water evaporates in the moving air, the temperature on the thermometer will drop. The amount the temperature drops helps to tell the amount of humidity in the air.
This method of humidity measurement is similar to when one goes swimming on a warm breezy day. Before entering the water, the temperature feels warm even with a breeze. If one jumps into the water and becomes wet and then gets back out, the breeze suddenly feels very cool. This is called evaporative cooling. Generally, the lower the humidity in the air, the bigger the drop in temperature.
Electric hygrometers work by measuring the electrical resistance of a particular substance. Most substances, such as lithium chloride, have varying resistances to an electrical current based on the humidity in the air. These differences are then calculated to display the humidity.
Chemical hygrometers work by using a chemical substance and exposing it to air. The chemical will be measured before being exposed and then again after. Any changes in weight indicate how much humidity is in the air.
Although hygrometers are fairly accurate methods of humidity measurement, the most accurate way of testing humidity on a global scale is through the use of satellites. Specialized satellites can detect the exact humidity in the troposphere, thereby giving accurate readings about weather conditions around the world. They are capable of tracking sudden changes in humidity and weather patterns to predict thunder storms, wind, and climate changes.