I use a psychrometer to measure the relative humidity on a starry summer night. My dry-bulb temperature is 18 degrees Celsius, and my
wet-bulb temperature is 19 degrees Celsius. What is the relative
humidity, in %?

Thermometers are used to measure temperature. Thermometers can also be used to find

relative humidity. Relative humidity is the amount of water vapor in air compared to the amount

of water vapor that the air can hold. The amount of water vapor that air can hold depends on

the temperature; the warmer the air, the more water vapor it can hold. Air is said to be saturated

when it cannot hold any more water.

One type of instrument used to find relative humidity is a psychrometer. A psychrometer

consists of two thermometers, with the bulb of one thermometer covered with a wet cloth.

Water evaporates from the wet cloth, which lowers the temperature. Relative humidity can

be found by subtracting the temperature on the wet-bulb thermometer from the temperature