Relative Humidity Air

Overview

Engineering reference data for Relative Humidity Air in air psychrometrics.

Key Formulas

Humidity Ratio

$\omega = 0.622 \frac{P_v}{P_a}$

Mass of water vapor per mass of dry air.

Relative Humidity

$\phi = \frac{P_v}{P_{vs}} \times 100\%$

Ratio of actual to saturation vapor pressure.

Wet Bulb Temperature

$T_{wb} = T_{db} - \frac{P_{vs} - P_v}{\gamma}$

Temperature measured by wet-bulb thermometer.

Enthalpy of Moist Air

$h = c_p T + \omega h_g$

Sensible + latent heat per unit mass of dry air.

Variables

Saturation Vapor Pressure Table

The saturation vapor pressure of water in air at various temperatures is critical for calculating relative humidity using the pressure method.

Unit Note: $10^{-3} \text{ bar} = 1 \text{ millibar (mbar)}$.

Altitude Correction Factors

Barometric pressure decreases with altitude, affecting the calculation of relative humidity. The following table provides correction factors (φ correction) for different altitudes.

Methods of Expression

Relative humidity ($\phi$ or $RH$) can be defined equivalently using three different ratios:

1. By Pressure: The ratio of the actual partial pressure of water vapor ($p_w$) to the saturation vapor pressure ($p_{ws}$) at the same dry-bulb temperature.
2. By Density: The ratio of the actual vapor density ($\rho_w$) to the saturation vapor density ($\rho_{ws}$) at the same dry-bulb temperature.
3. By Mass: The ratio of the actual mass of water vapor ($m_w$) in a given volume to the mass required to saturate that volume ($m_{ws}$) at the same temperature.

Interactive Charts

References

• Original Source