Which variable is NOT part of the equation for humidity ratio?

The humidity ratio, often represented as the mass of water vapor per unit mass of dry air, is calculated using a set of specific variables. Among the provided options, density is not an explicit part of the standard equation used to determine humidity ratio.

The formula commonly used for calculating the humidity ratio is:

[ W = \frac{0.622 \times P_v}{P - P_v} ]

In this equation:

• ( P ) represents the total atmospheric pressure.

• ( P_v ) denotes the partial pressure of the water vapor.

• The constant 0.622 provides a conversion factor that relates the water vapor to the dry air.

Density does not appear in this equation, as the formula focuses on the pressures involved rather than the density of the air or vapor itself. Therefore, while density is related to other aspects of air behavior and properties, it does not directly contribute to calculating the humidity ratio in this context. This makes density the correct choice as the variable that is not part of the humidity ratio equation.