Here's why:
* Psychrometer: A psychrometer uses two thermometers, one dry bulb and one wet bulb. The wet bulb thermometer is covered in a wick that is moistened with water.
* Evaporation: As water evaporates from the wet bulb, it cools the thermometer. The amount of cooling depends on the humidity of the air.
* High Humidity: When the relative humidity is high, there's already a lot of moisture in the air. This means the wet bulb thermometer won't cool much because there's less opportunity for evaporation.
* Low Humidity: When the relative humidity is low, there's less moisture in the air. This means the wet bulb thermometer will cool more significantly because there's more opportunity for evaporation.
Therefore, if the dry bulb and wet bulb temperatures are very similar, it indicates that the air is very humid and the rate of evaporation is low.
