Dew Point

Dew point (dew-point temperature, $t_d$) is the temperature to which air must be cooled (at constant humidity ratio $x$) for its relative humidity to reach 100 % — the air becomes saturated with water vapor.

Physical basis

Air can hold only a limited amount of water vapor, and the maximum depends on temperature — the warmer the air, the more vapor it holds. Cooled below its dew point, the excess water vapor condenses on the coldest available surface.

Dew point in the Mollier diagram

In the h–x diagram the dew point lies on the saturation curve (φ = 100 %) at the same value of $x$ as the starting air state. Graphically: from the air point, draw a vertical line down to the saturation curve.

Practical use

• Heat-recovery exchanger: below the dew point of the extract air, condensate forms on the plates/fins — drainage must be provided.
• Cooling with dehumidification: the coil must cool the air below its dew point to condense out the excess moisture.
• Thermal bridges: a surface colder than the dew point of the indoor air → risk of mold.

Formula (approximate)

For a quick estimate, use the Magnus approximation:

$$t_d \approx 243.04 \cdot \frac{\ln(\varphi/100) + 17.625\,t/(243.04+t)}{17.625 - \ln(\varphi/100) - 17.625\,t/(243.04+t)}$$

where $t$ is temperature [°C] and φ relative humidity [%]. For an accurate ASHRAE-based calculation, use PsychroView.