Psychrometer is used to measure
## Core Concept
A psychrometer is a type of thermometer used to measure the **humidity** of the air. It works on the principle that the evaporation of water from a surface is directly related to the humidity of the surrounding air. By comparing the readings of two thermometers, one wrapped in a wet cloth and the other exposed to the air, one can calculate the humidity.
## Why the Correct Answer is Right
The psychrometer is specifically designed to measure **relative humidity**, which is the ratio of the amount of water vapor actually present in the air to the greatest amount of water vapor the air could hold at the same temperature. This is achieved by using two thermometers: one is the dry-bulb thermometer, which measures the air temperature directly, and the other is the wet-bulb thermometer, which measures the temperature after being wrapped in a wet cloth. As water evaporates from the cloth, it cools the thermometer, and the difference in readings between the two thermometers allows for the calculation of relative humidity.
## Why Each Wrong Option is Incorrect
- **Option A:** This option is incorrect because, although a psychrometer does involve temperature measurements, it is not used to measure temperature directly but to calculate humidity from temperature differences.
- **Option B:** This option might seem plausible because a psychrometer does measure differences in temperature readings (which relate to humidity), but it is not used to measure air pressure.
- **Option C:** This option is incorrect because, while related to the environment, a psychrometer specifically measures humidity, not wind speed or direction.
## Clinical Pearl / High-Yield Fact
A key point to remember is that the **wet-bulb temperature** (the lowest temperature that can be reached by wrapping a thermometer in a wet cloth and placing it in the air stream) is an important measurement in heat stress calculations and can be a limiting factor for human tolerance to heat in humid environments.
## Correct Answer Line
**Correct Answer: C. Humidity**