Defining Specific Humidity in Air

The question asks for the term that describes the ratio of the mass of water vapour present in the air to the total mass of air, including the water vapour itself.

Analyzing Humidity Measures

Let's examine the given options:

• Mixing ratio: This is the ratio of the mass of water vapour to the mass of dry air. It doesn't include the mass of water vapour in the denominator.
• Relative humidity: This is a percentage comparing the actual amount of water vapour in the air to the maximum amount the air can hold at a specific temperature. It's not a direct mass ratio.
• Specific humidity: This is defined as the ratio of the mass of water vapour ($m_v$) to the total mass of moist air ($m_{total} = m_{dry} + m_v$). This perfectly matches the question's description. The formula is: $q = \frac{m_v}{m_{total}}$.
• Vapour pressure: This is the partial pressure exerted by water vapor molecules in the air. It's a measure of pressure, not a mass ratio.

Conclusion on Air Vapor Ratio

Based on the definitions, the term that best fits the description "Ratio of mass of water vapour actually present in the air to a unit mass of air, including water vapours" is Specific humidity.