EKS-HUM-003 / Moist-air state and psychrometric properties
Dew Point
露点
Dew point is the temperature at which moist air reaches saturation when cooled at constant pressure without changing its water-vapour content.
- Abbreviations
- DP
- Aliases
- dew-point temperature
QUANTITY, SYMBOL AND UNIT
Quantity / objectDew-point temperature
Symbolt_dp
SI°Cdegree Celsius
IP°Fdegree Fahrenheit
Definition
Dew-point temperature describes an air sample by the saturation temperature that corresponds to its existing water-vapour content. If the air is cooled below this temperature and a suitable surface is available, liquid-water condensation can begin.
Below the water triple-point region, the corresponding saturation condition is more accurately described as frost point when ice is the stable condensed phase.
Formula and variables
p_w = RH · p_ws(t_db); solve p_ws(t_dp) = p_w Conditions: RH is expressed as a fraction. The saturation-pressure relation must use the applicable water or ice branch and the same pressure basis.
| Symbol | Meaning | Unit |
|---|---|---|
p_w | water-vapour partial pressure | Pa or kPa |
RH | relative humidity as a fraction | 1 |
p_ws | saturation water-vapour pressure | Pa or kPa |
t_db | dry-bulb temperature | °C |
t_dp | dew-point temperature | °C |
Relations and distinctions
Dew point and relative humidity
Dew point tracks water-vapour content more directly. Relative humidity also changes when dry-bulb temperature changes, even if no moisture is added or removed.
Dew point and surface temperature
Dew point belongs to the air state; surface temperature belongs to the material or equipment surface. Their difference is used to screen condensation risk.
Engineering meaning
- Compare air dew point with the coldest credible surface temperature when condensation is the concern.
- Use dew point to compare moisture conditions across air streams whose dry-bulb temperatures differ.
- Retain sensor uncertainty and transient cold spots when turning a dew-point reading into a project margin.
Common misconceptions
MisconceptionA low relative-humidity reading always means a low condensation risk.
Accurate explanationRelative humidity is temperature-dependent. Warm air can show a moderate RH while still having a dew point above a cold surface.
MisconceptionThe dew point is the temperature of the coldest surface.
Accurate explanationDew point is an air-property temperature. The coldest surface is a separate measured or calculated condition.
Application boundaries
- Dew point alone does not provide moisture load, required airflow, equipment capacity or operating energy.
- A spot dew-point value does not represent door opening, weather changes, thermal bridges or start-up transients.
- The calculation is only as reliable as the temperature, humidity and pressure basis used for the air state.
Verified example
Illustrative condensation check
At 25°C dry-bulb and 60% RH, the calculated dew point is 16.70°C. A surface at 15°C is below that nominal dew point, so condensation is thermodynamically possible at the entered condition.
Basis: Calculated with the ASHRAE psychrometric saturation-pressure equations used by the Yakeclimate dew-point tool.Supporting sources
-
ASHRAE / SRC-ASHRAE-F25-CH01
ASHRAE Handbook - Fundamentals, Chapter 1: Psychrometrics ↗
2025 SI edition / Chapter 1, humidity parameters and moist-air relationships
Supports: Dew-point definition; Humidity parameters; Moist-air relationships -
National Institute of Standards and Technology / SRC-NIST-HUMIDITY-CAL
Humidity Calibration Services at NIST ↗
Updated 2025-08-14 / Description and calibration uncertainties
Supports: Dew/frost-point measurement; Relative-humidity measurement; Calibration uncertainty
Maintenance record
Dew Point
- Maintained by
- Yakeclimate Engineering Team
- Authoritative sources verified
- July 31, 2026
- Terminology checked
- July 31, 2026
- Last updated
- July 31, 2026