Input Values
Example Data Table
| Temperature (°C) | Relative Humidity (%) | Pressure (hPa) | e (hPa) | Dew Point (°C) | Absolute Humidity (g/m³) |
|---|---|---|---|---|---|
| 20 | 50 | 1013.25 | 11.69 | 9.26 | 8.64 |
| 25 | 60 | 1013.25 | 19.01 | 16.70 | 13.82 |
| 30 | 70 | 1000 | 29.71 | 23.93 | 20.29 |
Formula Used
1) Saturation vapor pressure
Over water (Magnus): es = 6.112 · exp(17.67T / (T + 243.5))
Over ice (Magnus): es = 6.112 · exp(22.46T / (T + 272.62))
2) Actual vapor pressure
e = (RH/100) · es where RH is relative humidity in percent.
3) Dew point
Using Magnus inversion: Td = 243.5·ln(e/6.112) / (17.67 − ln(e/6.112))
4) Absolute humidity
ρv = 216.7 · e / (T + 273.15) giving g/m³ when e is in hPa.
5) Mixing ratio and specific humidity
w = 0.622 e / (p − e) (kg/kg)
q = w / (1 + w) (kg/kg)
6) Vapor pressure deficit
VPD = es − e (hPa), clipped at zero.
How to Use This Calculator
- Enter air temperature and pick °C or °F.
- Enter relative humidity as a percentage from 0 to 100.
- Enter total air pressure and select its unit.
- Choose “Over ice” for cold, subzero conditions.
- Select density unit and rounding decimals for your report.
- Press Calculate to view results above the form.
- Use CSV or PDF buttons to export the computed results.
Professional Guide to Water Vapor Calculations
1) Why Water Vapor Matters in Physics
Water vapor is the most variable greenhouse gas and a major carrier of latent heat. In laboratories, HVAC design, meteorology, and combustion studies, accurate moisture properties improve energy budgets, sensor calibration, and safety decisions.
2) Inputs That Control Moisture State
This calculator uses air temperature, relative humidity, and total pressure. Temperature sets the saturation limit, humidity scales the actual amount present, and pressure affects mass ratios because dry air and vapor share the same total pressure. Built-in unit choices reduce conversion mistakes. Pressure input also helps validate results when e approaches p too closely.
3) Saturation Vapor Pressure and Temperature
Saturation vapor pressure es rises rapidly with temperature due to molecular kinetics. A small warming can produce a large increase in es, which is why warm air can contain far more moisture than cold air. For subzero conditions, selecting “over ice” is often more appropriate than “over water.”
4) Actual Vapor Pressure from Relative Humidity
Relative humidity is a ratio: RH = 100·e/es. The calculator converts RH into actual vapor pressure e, which is a true thermodynamic partial pressure. Outputs in hPa and Pa support both atmospheric practice and engineering specifications.
5) Dew Point for Condensation Risk
Dew point is the temperature where e equals es. If a surface cools below the dew point, condensation becomes likely. Engineers use dew point to assess fogging, corrosion risk, and insulation requirements in ducts, windows, and enclosures.
6) Absolute Humidity for Mass Balance
Absolute humidity (vapor density) expresses grams of water vapor per cubic meter. It is useful for chamber experiments, drying systems, and ventilation studies because it links directly to volumetric flow and water mass transport across boundaries. Choose g/m³ for reporting, kg/m³ for modeling work.
7) Mixing Ratio and Specific Humidity
Mixing ratio w compares vapor mass to dry-air mass, while specific humidity q compares vapor mass to total moist-air mass. These measures are convenient for atmospheric profiles, psychrometrics, and conservation equations. At lower ambient pressure, the same e yields higher w.
8) Vapor Pressure Deficit for Evaporation
VPD = es − e measures the drying demand of air. Higher VPD increases evaporation from surfaces and materials, often accelerating drying. Lower VPD indicates near-saturation conditions, reduced evaporation potential, and higher condensation likelihood. Use VPD to compare drying conditions across days.
FAQs
1) What is the difference between vapor pressure and relative humidity?
Vapor pressure is the actual partial pressure of water vapor. Relative humidity compares that vapor pressure to the saturation value at the same temperature, so it changes when temperature changes even if vapor pressure stays constant.
2) When should I choose the “over ice” option?
Use it when air temperature is below 0 °C or when you expect ice surfaces to control saturation. It uses an ice-based saturation relationship that better represents cold, dry environments.
3) Why does pressure affect mixing ratio and specific humidity?
Mixing ratio depends on e/(p−e). For the same vapor pressure, lower total pressure increases the ratio, which matters at altitude or in low-pressure chambers.
4) Is dew point always lower than air temperature?
Usually, yes. Dew point equals air temperature only at 100% relative humidity. If your inputs produce dew point above air temperature, check units or humidity value.
5) Which output should I use for ventilation calculations?
Absolute humidity (g/m³ or kg/m³) is often best because it pairs directly with volumetric airflow to estimate water mass removal or addition over time.
6) What does a high VPD mean in practice?
High vapor pressure deficit indicates strong drying potential, so evaporation and drying rates generally increase. Low VPD indicates near-saturation air and reduced drying.
7) How accurate are the results?
The calculator uses widely used Magnus-type equations suitable for typical environmental ranges. For extreme temperatures, high pressures, or precision metrology, compare with a standards-based formulation used in your domain.