Here's why:
* Psychrometers work by measuring the difference between the wet-bulb temperature (measured by a thermometer with a wet wick) and the dry-bulb temperature (measured by a standard thermometer).
* Evaporation is the key: The wet wick cools down as water evaporates from it. The faster the evaporation, the lower the wet-bulb temperature.
* Low humidity means less water vapor in the air. This allows for faster evaporation from the wet wick, resulting in a larger temperature difference between the wet and dry bulbs.
* High humidity means more water vapor in the air. This slows down evaporation, making the wet-bulb temperature closer to the dry-bulb temperature.
Therefore, when the two temperatures on a psychrometer are nearly the same, it indicates that the air is very dry and the relative humidity is low.
