Step 1: Understanding the Concept:
Humidity can be expressed in various ways depending on whether the mass of water vapor is compared to total air mass or just the dry component.
Step 2: Key Formula or Approach:
The definition of Mixing Ratio ($w$) is:
\[ w = \frac{m_v}{m_d} \]
Where:
\( m_v \) = mass of water vapor.
\( m_d \) = mass of dry air.
Step 3: Detailed Explanation:
- Specific Humidity ($q$): Ratio of mass of water vapor to the total mass of the air parcel ($m_v / (m_v + m_d)$).
- Relative Humidity (RH): Ratio of actual vapor pressure to saturation vapor pressure (expressed as %).
- Mixing Ratio ($w$): As defined above, comparing vapor specifically to the dry portion. It is a conserved quantity in a parcel during adiabatic ascent/descent as long as no condensation occurs.
Step 4: Final Answer:
The ratio of mass of water vapor to the mass of dry air is the Mixing Ratio.