Temperature Relative Humidity

Overview

Engineering reference data for Temperature Relative Humidity 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

Heating Process Analysis

When cold outdoor air is heated indoors at constant specific humidity, the relative humidity decreases. The relationship can be expressed as:

$\phi_{\text{indoor}} = \phi_{\text{outdoor}} \times \frac{P_{ws}(T_{\text{outdoor}})}{P_{ws}(T_{\text{indoor}})}$

where $\phi$ is relative humidity, $P_{ws}$ is saturation vapor pressure at temperature $T$. As indoor temperature increases, $P_{ws}$ increases, leading to lower relative humidity.

This process is visualized in a Mollier diagram or psychrometric chart as a horizontal line from point A to point B, representing constant specific humidity.

Effect of Indoor Moisture Sources

The calculations above assume no additional moisture is introduced indoors. In practice, latent heat from occupants, animals, or production processes adds water vapor, increasing the indoor relative humidity. This shifts the process line to the right in the Mollier diagram. Sensible heat from these sources further increases the temperature, shifting the point upward, potentially altering the final indoor humidity.

Reference Data Table

References

• Original Source