Vapor Pressure Calculator From Temperature and Wet Bulb
Compute actual vapor pressure, relative humidity, dew point, and vapor pressure deficit from dry-bulb and wet-bulb measurements.
Equation used: e = es(Tw) – A(1 + 0.00115Tw)P(T – Tw)
Expert Guide: Calculating Vapor Pressure From Temperature and Wet Bulb
Calculating vapor pressure from dry-bulb and wet-bulb temperature is one of the most practical psychrometric skills in environmental science, agriculture, HVAC design, meteorology, and industrial process control. A wet-bulb reading captures the cooling effect of evaporation, and that effect is directly connected to how much moisture the air can still absorb. By combining dry-bulb temperature, wet-bulb temperature, and atmospheric pressure, you can estimate the actual vapor pressure of water in air with very good field accuracy.
Why does this matter in practice? Because vapor pressure is more physically meaningful than relative humidity alone. Relative humidity changes dramatically when temperature changes, even if moisture content does not. Vapor pressure gives you a direct measure of moisture partial pressure and lets you compare conditions between locations and times more reliably. It is also the basis for derived indicators such as dew point and vapor pressure deficit (VPD), both of which are critical in crop irrigation scheduling, heat stress assessment, and mold risk evaluation in buildings.
Core Concepts You Need Before Doing the Math
- Dry-bulb temperature (T): the actual ambient air temperature.
- Wet-bulb temperature (Tw): temperature measured with a wetted wick and airflow; usually lower than dry-bulb unless air is saturated.
- Saturation vapor pressure (es): maximum vapor pressure air can hold at a specific temperature.
- Actual vapor pressure (e): current partial pressure of water vapor in the air.
- Psychrometric coefficient: correction that links evaporative cooling to pressure and ventilation conditions.
When dry-bulb and wet-bulb are far apart, the air is drier and evaporation is stronger. When they are close, air is more humid. If both are equal, the air is saturated and relative humidity is about 100%.
The Working Formula
A commonly used equation for ventilated psychrometers is:
e = es(Tw) - A(1 + 0.00115Tw)P(T - Tw)
where:
e= actual vapor pressure (kPa)es(Tw)= saturation vapor pressure at wet-bulb temperature (kPa)A= psychrometer constant base (often 0.00066 for aspirated instruments)P= atmospheric pressure (kPa)TandTwin °C
To compute saturation vapor pressure you can use Tetens-style approximation:
es(T) = 0.6108 × exp((17.27T)/(T + 237.3))
Step-by-Step Procedure
- Measure dry-bulb and wet-bulb temperatures with a calibrated psychrometer.
- Convert readings to Celsius if necessary.
- Get local atmospheric pressure in kPa (or estimate from station elevation).
- Compute
es(Tw)using a saturation equation. - Apply psychrometric correction term
A(1 + 0.00115Tw)P(T - Tw). - Subtract correction from
es(Tw)to get actual vapor pressuree. - Optionally compute:
- Relative humidity =
100 × e / es(T) - VPD =
es(T) - e - Dew point from inverse saturation equation.
- Relative humidity =
Reference Statistics: Saturation Vapor Pressure by Temperature
The table below uses standard psychrometric approximations and aligns with values commonly seen in meteorological references.
| Temperature (°C) | Saturation Vapor Pressure es (kPa) | Saturation Vapor Pressure es (hPa) |
|---|---|---|
| 0 | 0.611 | 6.11 |
| 10 | 1.228 | 12.28 |
| 20 | 2.338 | 23.38 |
| 25 | 3.168 | 31.68 |
| 30 | 4.243 | 42.43 |
| 35 | 5.623 | 56.23 |
| 40 | 7.375 | 73.75 |
Comparison Scenarios Using Dry-Bulb and Wet-Bulb
These example calculations assume pressure of 101.3 kPa and aspirated psychrometer constant. They illustrate how widening dry-wet spread sharply lowers humidity and increases vapor pressure deficit.
| Dry-Bulb (°C) | Wet-Bulb (°C) | Estimated Actual Vapor Pressure e (kPa) | Relative Humidity (%) | VPD (kPa) |
|---|---|---|---|---|
| 30 | 28 | 3.53 | 83 | 0.71 |
| 30 | 24 | 2.66 | 63 | 1.58 |
| 30 | 20 | 1.84 | 43 | 2.40 |
| 35 | 25 | 2.43 | 43 | 3.19 |
How to Interpret the Result Correctly
If your calculated vapor pressure is high, the air carries more moisture and evaporation slows. If vapor pressure is low, air is moisture-hungry and evaporation accelerates from soil, leaves, skin, and wet surfaces. This is exactly why farmers monitor VPD: plant transpiration is driven by the gap between saturation at leaf temperature and actual atmospheric vapor pressure. HVAC teams also rely on vapor pressure and dew point to prevent condensation inside ducts and building assemblies.
A common mistake is focusing on relative humidity alone. For example, 60% RH at 15°C and 60% RH at 30°C represent very different moisture conditions, because saturation vapor pressure rises nonlinearly with temperature. At 30°C, 60% RH implies much higher actual vapor pressure than at 15°C. Always examine absolute moisture indicators when precision matters.
Instrument and Field Quality Tips
- Ensure good airflow over the wet wick for valid wet-bulb depression.
- Use clean distilled water on the wick to reduce contamination effects.
- Shield thermometers from direct solar radiation.
- Allow readings to stabilize before recording values.
- Use pressure corrected for local elevation when possible.
- Repeat measurements and average if conditions are turbulent.
In low-wind, high-radiation environments, naturally ventilated psychrometers can bias wet-bulb upward, leading to overestimated humidity. That is why aspirated systems and accurate psychrometric constants are preferred for quality-control work.
Applications Across Sectors
Agriculture: Vapor pressure and VPD guide greenhouse misting, irrigation intervals, and disease prevention strategies. Many crops perform best within target VPD bands because gas exchange and stomatal behavior are strongly humidity dependent.
Meteorology: Surface humidity analyses, fog forecasting, and severe weather diagnostics use dew point and vapor pressure fields derived from psychrometric relationships.
HVAC and buildings: Dew point control is central to latent load management, indoor comfort, and condensation avoidance on chilled surfaces.
Occupational safety: Heat stress frameworks often combine temperature and humidity conditions where vapor pressure informs evaporative cooling potential for workers.
Authoritative Technical References
For deeper validation and standards-level methods, review:
- U.S. National Weather Service: Vapor Pressure and Humidity Calculations (.gov)
- NOAA JetStream: Humidity Fundamentals (.gov)
- Penn State Extension: Relative Humidity and Vapor Pressure Deficit (.edu)
Practical Bottom Line
If you have dry-bulb temperature, wet-bulb temperature, and pressure, you have enough information to estimate actual vapor pressure reliably for most operational needs. From there, you can quickly derive RH, dew point, and VPD for decisions in agriculture, weather interpretation, industrial drying, and building moisture control. The calculator above automates this workflow while keeping the equation transparent so you can audit the numbers and explain them to clients, field teams, and students.
Values are approximate and intended for engineering and environmental estimation. For regulatory or laboratory-grade reporting, use calibrated instruments and method-specific standards.