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

Symbol	Description	Unit
$\omega$	Humidity ratio	kg/kg
$\phi$	Relative humidity	%
$P_v$	Vapor pressure	Pa
$P_{vs}$	Saturation vapor pressure	Pa
$T_{db}$	Dry bulb temperature	°C
$T_{wb}$	Wet bulb temperature	°C

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