Climatology – Moist air measures In atmospheric science, what is the term for the ratio of the mass (weight) of water vapour to the total mass (weight) of moist air, i.e., dry air plus the contained water vapour?

Introduction / Context:
This question assesses fundamental terminology used in meteorology and climatology to quantify atmospheric moisture. Different humidity measures answer slightly different questions about moisture content, so precision in definitions is essential for weather analysis, air-conditioning design, and environmental studies.

Given Data / Assumptions:

• The required quantity compares water vapour mass to the total mass of moist air (dry air + water vapour).
• Standard humidity terms include specific humidity, mixing ratio, relative humidity, and absolute humidity.
• We assume standard meteorological definitions.

Concept / Approach:
Specific humidity (often symbol q) is defined as mass of water vapour divided by total mass of moist air. The mixing ratio (often w or r) differs: it is mass of water vapour divided by mass of dry air only. Relative humidity relates the actual vapour content to saturation at the same temperature, and absolute humidity is mass of vapour per unit volume of air.

Step-by-Step Solution:
Identify the denominator stated in the problem: total mass of moist air.Match the definition: specific humidity = m_vapour / (m_dry_air + m_vapour).Contrast with mixing ratio: m_vapour / m_dry_air (not what is asked).Eliminate relative and absolute humidity because they use different bases (saturation ratio and volume basis respectively).

Verification / Alternative check:
On a psychrometric chart, specific humidity (or humidity ratio by some engineering conventions) is plotted to show moisture per total air mass basis; definitions in meteorology explicitly use total mass in the denominator for specific humidity.

Why Other Options Are Wrong:

• mixing ratio: uses mass of dry air as denominator, not total moist air.
• relative humidity: ratio of actual vapour pressure to saturation vapour pressure, temperature dependent.
• absolute humidity: mass of vapour per unit volume, not a mass fraction of total air.

Common Pitfalls:
Confusing specific humidity with mixing ratio due to similar names; always check the denominator. Also, do not treat relative humidity as a measure of moisture content alone since it depends strongly on temperature.

Final Answer:
specific humidity