Here's why:
* How a psychrometer works: A psychrometer uses two thermometers: a dry bulb and a wet bulb. The wet bulb thermometer has a wick covering its bulb, which is moistened with water.
* Evaporation and cooling: As water evaporates from the wet bulb, it absorbs heat from the surrounding air, causing the wet bulb temperature to drop.
* Relative humidity: The difference in temperature between the dry bulb and wet bulb thermometers indicates the rate of evaporation, which is directly related to the relative humidity. If the difference is zero (meaning both thermometers read the same), it means there is no evaporation.
* 100% relative humidity: No evaporation means the air is already saturated with moisture, hence the relative humidity is 100%.
Let me know if you'd like more details on how psychrometers work!
