## Core Concept
A sling psychrometer is a device used in meteorology and physics to measure the **humidity** of air. It works on the principle that the evaporation of water from a wet cloth wrapped around one of the thermometers causes a cooling effect, which is directly related to the humidity of the surrounding air.

## Why the Correct Answer is Right
The sling psychrometer consists of two thermometers mounted on a rotating frame. One thermometer is wrapped in a wet cloth, while the other is exposed to the air. As the device is whirled, the water on the wet cloth evaporates, cooling the thermometer wrapped in the cloth. The difference in readings between the two thermometers is used to determine the **relative humidity** of the air. This is based on the psychrometric chart, which relates the readings of the two thermometers to the relative humidity.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is incorrect because while a sling psychrometer does involve temperature measurements, it is specifically designed to calculate humidity, not just measure temperature.
- **Option B:** This option might seem plausible since atmospheric pressure is crucial in many meteorological measurements. However, a sling psychrometer is not used to measure pressure but to calculate humidity based on temperature differences.
- **Option C:** This option is incorrect because a sling psychrometer measures the relative humidity, not the air velocity. Air velocity might influence the rate of evaporation but is not what the device measures directly.

## Clinical Pearl / High-Yield Fact
A memorable point for students is that the **wet bulb temperature** (the temperature reading of the thermometer wrapped in wet cloth) and the **dry bulb temperature** (the reading of the exposed thermometer) are essential for calculating the heat index and understanding the body's ability to cool itself through sweating. This concept is critical in understanding heat-related illnesses.

## Correct Answer: D. Relative Humidity.