Activity Overview: Temperature is the weather metric most obviously associated with human comfort, but humidity is the real key. Humidity is the amount of moisture in the form of water vapor in the air all around us, and it really affects how we feel in the weather. Meteorologists use the term relative humidity, which refers to the ratio of water vapor actually in the air compared to how much water vapor air can hold. In other words, fog is 100% relative humidity; the air contains as much water it can hold.
The instrument used to measure relative humidity is the hygrometer. A variety of different digital and analog models exist, but you can build a simple version with your class. Known as the sling psychrometer, this hygrometer takes a measurement with a "wet bulb" thermometer and a "dry bulb" thermometer simultaneously. The difference between the measurements can be interpreted to determine the relative humidity, because evaporation over the wet bulb is directly related to how much moisture is in the air.
First, wet the cotton gauze with water. The gauze should be damp, but not dripping. Cover the bulb of one thermometer with the wet gauze, and secure the gauze to the thermometer with the rubber band. [NOTE: Some thermometers use liquid mercury, which is toxic. Take special care when working with mercury-based thermometers. If you only have these thermometers to work with, consider performing this part of the procedure as the instructor and then only allowing students to observe.] The gauze must be moist with water in order for this activity to work.
Place both thermometers next to each other under the same conditions. Use a folder, magazine, or piece of cardboard to blow air over the thermometers. The fanning will not significantly change the temperature of the air, but it will catalyze evaporation, if possible. Wait several minutes for the thermometers to adjust to the temperature of the air. Water will evaporate from the gauze if relative humidity allows, slightly cooling the wet bulb thermometer and allowing a temperature difference to show between the two.
Ask students to observe the temperature of the two thermometers and record the results. Then, consult the following chart to interpret the measurements from the thermometer in order to determine the relative humidity of your environment. For example, if the dry-bulb temperature is 24°C and there is a 2°C temperature difference between wet-bulb and dry-bulb temperature, then your environment has 84% relative humidity.