It's Not the Heat, It's the Humidity
The most common instrument for measuring relative humidity is the sling psychrometer. It consists of two thermometers mounted next to each other. One thermometer has a wet wick attached to its bulb, and the other is dry. The wick is moistened with water at air temperature. The two thermometers are attached to a handle, which is swung around. Water evaporates off the moist wick, causing a lowering of temperature on that bulb—called, for obvious reasons, the wet bulb.
Eventually no additional water will be able to evaporate into the air. The temperature stops going downward, and the resulting reading is called the wet bulb temperature. This is compared with the dry bulb reading. The difference is proportional to the dryness of the air. A large difference indicates that a large volume of water was able to evaporate. A smaller difference means that the air was already filled with water vapor and couldn't hold much more. The air was more humid. The following figure shows a classic sling psychrometer. Tables are then used to determine the relative humidity based on the wet and dry bulb readings. The dew point can be similarly determined.
Sometimes the instrument isn't swung around, and a fan blows air across. That is called an aspiration psychrometer.
A sling psychrometer is an instrument that measures relative humidity. It has two thermometers—a dry bulb and a wet bulb—mounted side by side and has a handle at one end. You whirl the instrument to get the readings.
An aspiration psychrometer is similar to a sling psychrometer, but rather than being whirled, air is drawn past it with an electric fan.
In addition to thermometers that whirl, there are other devices that determine the relative humidity. You've probably noticed that hair responds to changes in humidity. When that humidity increases, so too does the length of hair. If the hair is naturally curly, it curls more—it's time for the frizzies. If the hair is naturally straight, it tends to go limp.
The relationship between hair length and humidity is used in an instrument called a hair hygrometer. Strands of naturally blonde hair or horse hair with oils removed are attached to a system of levers. These amplify the change in hair length, and then cause a dial to move. Sometimes the dial becomes an arm on a rotating drum, and a record of the humidity can be determined. The hair hygrometer is not as accurate as a sling psychrometer, and it requires considerable calibration, especially when there are large variations in humidity.
More modern instruments are electrical and are based on the principle that electrical resistance changes for some materials when exposed to differences in moisture. One type of electric hygrometer uses carbon coating on a flat plate. The current depends on how much moisture is present on the plate. Other devices use a lithium chloride solution from which water is evaporated by the passage of an electrical current.