These thermometers break easily and should be handled carefully. Tthey should also be cleaned daily of outside elements such as dust, dirt and moisture so that the most accurate temperatures are recorded. In order to get the minimum and maximum temperature for each day, a student should record the temperature early in the morning and at the end of the school day. The amount of moisture in the air is usually stated in terms of relative humidity. This term refers to the percentage of moisture the air holds relative to the amount it can hold at a particular temperature.
Relative humidity is measured using a instrument called a psychrometer.
A psychrometer is composed of two thermometers a wet-bulb thermometer and a regular thermometer. The wet-bulb thermometer in actuality is a regular thermometer. However, the bulb of the thermometer is covered with a piece of wet cloth. The other thermometer is called a dry-bulb thermometer. The two thermometers are attached to each other and swung around in the outside air.