Saturation Vapor Pressure and Temperature
Calculate saturation vapor pressure consistently and explain why temperature errors change RH, dew-point, and VPD interpretations.
Educational reference · evidence, sources, and limits shown below
Calculate saturation vapor pressure consistently and explain why temperature errors change RH, dew-point, and VPD interpretations.
Terms to know
- Saturation vapor pressure
- Equilibrium vapor pressure of water over a flat liquid-water surface at a stated temperature.
- Actual vapor pressure
- Partial pressure contributed by water vapor in the measured air.
- Tetens-type equation
- Common exponential approximation for saturation vapor pressure over liquid water.
- Psychrometrics
- Study of thermodynamic properties of moist air.
Core science
Saturation vapor pressure rises nonlinearly as temperature rises. A practical horticultural approximation for temperature T in degrees Celsius is e_s(T) = 0.6108 x exp[17.27T/(T + 237.3)], yielding kilopascals over liquid water. Other valid formulas use slightly different constants and temperature ranges, so calculation method must be recorded.
Actual vapor pressure can be estimated as RH/100 x e_s(T_air) when temperature and RH describe the same air parcel. Because e_s changes rapidly with temperature, a small temperature bias changes derived vapor pressure, VPD, and dew point. Mixing an RH reading from one location with temperature from another creates a number that may describe neither location.
Saturation does not mean air has a fixed maximum mass of water independent of conditions. Vapor pressure is the more precise language. Heating an air parcel without adding or removing moisture raises e_s and lowers RH when pressure changes are negligible; cooling does the reverse until saturation and possible condensation.
Why this matters in cultivation
- Standardize one approved formula, units, rounding rule, and source-temperature pair across dashboards and logs. Confirm whether a controller reports kPa, hPa, mbar, or another unit before comparing values.
- Recalculate a few logged values independently during sensor audits. A polished VPD chart can still be wrong because of Fahrenheit-to-Celsius errors, percent-versus-decimal RH, mismatched sensor locations, or using leaf temperature in only part of the formula.
Measure and record
Inputs
Temperature value and unit, RH, exact location, time, and sensor IDs.
Formula
Equation, constants, liquid-water or ice convention where relevant, and software version.
Outputs
Saturation vapor pressure, actual vapor pressure, dew point or VPD, units, and rounding.
Independent check
Hand calculation or second calculator result and acceptable tolerance.
Exception
Missing data, out-of-range input, suspected sensor error, and disposition.
Common misconceptions
Correction: The useful relationship is between temperature and equilibrium vapor pressure, not a literal holding capacity.
Correction: Equations, constants, leaf assumptions, units, and rounding can differ.
Correction: The paired readings must represent the same air mass and time.
Evidence limits
The stated equation is a practical approximation over ordinary cultivation temperatures, not a universal thermodynamic standard for every pressure, ice condition, or extreme temperature.
Related encyclopedia topics
- THC-ENC-083, THC-ENC-085-087, THC-ENC-099, and environmental-data validation.
Source notes
- Shamshiri R.R. et al. (2018). Review of optimum temperature, humidity and vapour pressure deficit for greenhouse microclimate evaluation and control. International Agrophysics 32:287-302.
- Michigan State University Extension (2015). Why Should Greenhouse Growers Pay Attention to Vapor-Pressure Deficit?
- University of Massachusetts Extension (2016). Managing Light, Temperature, and Relative Humidity in Greenhouses.
This lesson summarizes the source material and its evidence limits for education. Use direct measurement, controlled comparison, and the cited sources when conditions differ or a decision carries meaningful risk.