**Question:** Sling psychrometer is used for measuring –

A. Relative Humidity
B. Air Pressure
C. Air Velocity
D. Air Temperature

**Correct Answer:** A. Relative Humidity

**Core Concept:** A psychrometer is a device used to measure humidity in the atmosphere. It consists of two wet bulbs and one dry bulb thermometer. The wet bulbs are covered with moistened fibers, which helps to evaporate moisture from the surface and reduce the temperature of the bulb. The dry bulb measures the ambient temperature.

**Why the Correct Answer is Right:** A Sling Psychrometer, also known as an aneroid psychrometer, is designed to measure relative humidity. The relative humidity is calculated based on the difference in temperature between the wet and dry bulbs, as well as the ratio of the wet bulb temperature to the dry bulb temperature. Relative humidity is an essential parameter in meteorology, agriculture, and climate studies, as it indicates the amount of moisture available in the air compared to the maximum amount of moisture the air can hold at a given temperature.

**Why Each Wrong Option is Incorrect:**

**Option B (Air Pressure):** The psychrometer measures relative humidity, not air pressure. Air pressure is determined by the atmospheric conditions and is measured using a barometer or an altimeter.

**Option C (Air Velocity):** The psychrometer is not designed to measure air velocity. Air velocity can be measured using an anemometer, which is an instrument specifically designed to measure wind speed or airflow rate.

**Option D (Air Temperature):** While the psychrometer does measure air temperature, it is not the primary purpose of the device. The main focus is on determining relative humidity, which is calculated using the wet and dry bulb temperatures.

**Clinical Pearl:** Relative humidity is a crucial parameter for understanding the comfort and health conditions of individuals. High humidity levels can lead to increased risk of skin irritation, respiratory issues, and heat-related illnesses like heat exhaustion and heatstroke. On the other hand, low humidity can lead to dryness, chapped skin, and respiratory problems. Maintaining a balanced relative humidity level is essential for optimal living and working conditions.

In conclusion, the correct answer, A (Relative Humidity), is the primary purpose of a sling psychrometer, while the incorrect options are related to other aspects of weather measurement (B: Air Pressure) or are not directly measured by the psychrometer (C: Air Velocity, D: Air Temperature)