In meteorology, the amount of water vapour present in the air at a given time is called what?

Introduction / Context:
When we talk about weather, we often mention that the air feels dry or very sticky and moist. This feeling is related to the amount of water vapour present in the atmosphere, which is a key factor in weather prediction and human comfort. The term used to describe this quantity is extremely common in weather reports and basic science.

Given Data / Assumptions:

• We are discussing the gaseous form of water present in the atmosphere.
• The focus is on how much water vapour the air contains at a given moment.
• We are not talking about liquid water drops like rain or visible dew on surfaces.
• The question asks for the correct meteorological term for this quantity.

Concept / Approach:
Water vapour is the invisible gaseous form of water in the atmosphere. The measure of how much water vapour is in the air is called humidity. Humidity can be expressed in various ways, such as absolute humidity or relative humidity, but at general knowledge level the single word humidity represents the amount of water vapour in the air. Dew, by contrast, is liquid water condensed on surfaces, and foam is a mixture of gas in liquid; neither describes how much vapour is in the air as a whole.

Step-by-Step Solution:
Step 1: Identify that the question refers to water vapour, not liquid water. Step 2: Recall that weather reports often mention phrases like high humidity or low humidity to describe how moist the air is. Step 3: Recognise that humidity specifically refers to the amount of water vapour present in the air at a given time. Step 4: Compare the options and see that hot air, dew and foam do not describe this quantity. Step 5: Conclude that humidity is the correct scientific and everyday term.

Verification / Alternative check:
Science textbooks define humidity as the measure of the water vapour content of the air. Devices called hygrometers measure humidity. Dew forms when air cools and water vapour condenses on surfaces, which is a result of humidity reaching saturation, but dew itself is not the name of the vapour content. The other options are clearly unrelated to this definition, confirming that humidity is the only correct answer.

Why Other Options Are Wrong:
Hot air: This just describes temperature, not the amount of water vapour.

Dew: Dew is liquid water deposited on surfaces, not the overall vapour content of the air.
Foam: Foam is a mixture of gas bubbles in a liquid, seen in soaps or waves, and has nothing to do with atmospheric water vapour.

Common Pitfalls:
Some learners confuse humidity with dew point, which is the temperature at which air becomes saturated and dew forms. Others think that humidity refers only to how the air feels to humans, but scientifically it always refers to water vapour content. Remember that whenever you read about moist or sticky air in weather contexts, the technical word behind it is humidity.

Final Answer:
The amount of water vapour in the air is called Humidity.