Function moist_speed_of_sound 

pub fn moist_speed_of_sound(
    temp_k: f64,
    pressure_pa: f64,
    humidity_percent: f64,
) -> f64

Speed of sound using an ideal-gas moist-air mixture approximation, first order in the water-vapor mole fraction.

The water-vapor mole fraction is derived from the SAME IAPWS saturation vapor pressure (enhanced_saturation_vapor_pressure) and CIPM-2007 enhancement factor used by calculate_air_density_cimp, so a single vapor formula feeds both density and c. Only the vapor fraction is shared with the CIPM path; the acoustic relation itself remains ideal-gas. The mixture applies first-order humidity corrections to the dry-air heat-capacity ratio and gas constant, then evaluates sqrt(gamma * R * T). It is not Cramer’s full real-gas polynomial and does not include Cramer’s pressure or carbon-dioxide terms.

Arguments

• temp_k - Temperature in Kelvin
• pressure_pa - Total (station) pressure in Pa
• humidity_percent - Relative humidity percentage (0-100)

Returns

Speed of sound in m/s