In meteorology, which physical quantity is specifically defined as a measure of the moisture content or water vapour present in air?

Introduction / Context:
This question again relates to atmospheric science and focuses on the concept used to describe how moist the air is. People often say that the weather feels humid or dry. Scientists and weather reports use a specific term to quantify this moisture content. Identifying that term is important for understanding weather forecasts and comfort levels in different climates.

Given Data / Assumptions:

• We are dealing with the air in the atmosphere.
• The physical property of interest is the moisture content or water vapour present in the air.
• Options include density, viscosity, impurity and humidity.
• We assume standard meteorological definitions.

Concept / Approach:
Humidity is the term used in meteorology to describe the amount of water vapour in the air. It can be expressed as absolute humidity, relative humidity or specific humidity, but the general concept always refers to moisture content. Density is mass per unit volume of air, viscosity is a measure of internal friction in fluids and impurity is not a standard physical quantity in this context. Therefore, humidity is the correct answer.

Step-by-Step Solution:
Step 1: Recognise that moisture content in air is associated with water vapour, the gaseous form of water. Step 2: Identify that meteorologists use the word humidity to describe how much water vapour is present. Step 3: Recall that relative humidity is commonly reported in weather reports as a percentage, showing how close the air is to saturation. Step 4: Understand that density is mass per unit volume and can describe air in general but does not specifically refer to moisture content. Step 5: Note that viscosity describes a fluid resistance to flow and is not directly a measure of water vapour content. Step 6: Realise that impurity is a generic idea of contamination and is not a precisely defined thermodynamic quantity for moisture. Step 7: Conclude that humidity is the correct quantity measuring moisture in air.

Verification / Alternative check:
Weather apps and forecasts regularly include relative humidity values along with temperature and pressure. Devices that measure moisture in air are called hygrometers, and they output humidity readings. Air conditioning systems often control humidity to maintain comfort indoors. All these examples confirm that humidity is indeed the standard term that quantifies water vapour content in air, distinguishing it clearly from density and viscosity.

Why Other Options Are Wrong:
Density is the mass per unit volume of air or any substance and does not specifically describe the water vapour fraction. Viscosity measures the internal friction or resistance to flow in fluids and does not give a direct measure of moisture content. Impurity is a vague term for contamination and is not a standard physical quantity used to describe moisture in atmospheric science.

Common Pitfalls:
Students sometimes confuse humidity with density, thinking that moist air must always be denser, but humidity itself is a separate concept. Another source of confusion is mixing up humidity with precipitation. Precipitation involves liquid or solid water falling from clouds, while humidity refers to invisible water vapour in the air. Remember that devices called hygrometers measure humidity, reinforcing the connection between the term humidity and moisture in air.

Final Answer:
The physical quantity that measures the moisture present in air is Humidity.