EKS-HUM-004 / Moist-air state and psychrometric properties
Frost Point
霜点
Frost point is the temperature at which water vapour in a gas is in equilibrium with a plane surface of ice at the stated pressure.
- Abbreviations
- FP
- Aliases
- frost-point temperature
QUANTITY, SYMBOL AND UNIT
Quantity / objectFrost-point temperature
Symbolt_fp, T_FP
SI°Cdegree Celsius
SIKkelvin
IP°Fdegree Fahrenheit
Definition
Frost point is the temperature at which water vapour in a gas is in equilibrium with a plane surface of ice at the stated pressure.
It is the ice-phase counterpart to dew point, which is defined with liquid water. At low humidity, instruments and calculations must identify whether the reported saturation temperature is referenced to ice or to liquid water.
Relations and distinctions
Frost point and dew point
Frost point uses saturation with respect to ice; dew point uses saturation with respect to liquid water. Their numerical values are not interchangeable below the freezing region.
Frost point and air temperature
Frost point expresses water-vapour content on an ice-saturation basis. Air temperature states the current thermal condition.
Engineering meaning
- Use the phase basis stated by the instrument, standard or calculation when specifying very dry air.
- Record gas pressure and measurement uncertainty when a low frost-point requirement is used for acceptance.
Common misconceptions
MisconceptionAny negative dew-point reading can be relabelled as frost point.
Accurate explanationThe condensed phase used in the saturation definition changes the result; the reporting basis must remain explicit.
Application boundaries
- A frost-point value does not by itself specify air temperature, RH or total gas pressure.
- A surface below the reported frost point may permit frost formation, but actual formation also depends on local temperature, nucleation and transient conditions.
Supporting sources
-
National Institute of Standards and Technology / SRC-NIST-SP250-83R1
Calibration of Hygrometers with the Hybrid Humidity Generator ↗
NIST Special Publication 250-83 Revision 1 / Section 3, Humidity Definitions, page 8
Supports: Frost-point temperature is defined by equilibrium between water vapour and ice at the stated gas pressure; Frost point and dew point use different condensed-water phases; Pressure belongs to a traceable frost-point determination -
National Institute of Standards and Technology / SRC-NIST-HYGROMETERS
Hygrometers ↗
Updated 2022-11-28 / Calibration scope and dew/frost-point measurement range
Supports: Frost point is a recognized humidity measurand; Hygrometers may be calibrated in dew/frost-point temperature
Maintenance record
Frost Point
- Maintained by
- Yakeclimate Engineering Team
- Authoritative sources verified
- August 3, 2026
- Terminology checked
- August 3, 2026
- Last updated
- August 3, 2026