Calculator
Formula Used
This calculator estimates saturation vapor pressure es from temperature. It outputs a pressure representing equilibrium between water vapor and a flat surface of water or ice.
Here, T is temperature in °C. Internally, values are converted to Pa for consistent unit conversion.
How to Use This Calculator
- Enter the air temperature and select its unit.
- Choose whether saturation is over water or over ice.
- Select a formula model if you need a specific standard.
- Pick your preferred output pressure unit.
- Click Calculate to view results above the form.
- Use Download CSV or Download PDF after calculation.
Example Data Table
| Temperature (°C) | Model | Phase | es (kPa) | es (hPa) |
|---|---|---|---|---|
| -10 | Magnus | Water | 0.28677 | 2.868 |
| 0 | Magnus | Water | 0.61094 | 6.109 |
| 10 | Magnus | Water | 1.22602 | 12.260 |
| 20 | Magnus | Water | 2.33344 | 23.334 |
| 30 | Magnus | Water | 4.23665 | 42.367 |
Article
1) What saturation vapor pressure means
Saturation vapor pressure, es, is the maximum partial pressure of water vapor that air can hold at a given temperature when in equilibrium with a water or ice surface. When actual vapor pressure equals es, the air is saturated and relative humidity is 100%.
2) Why temperature dominates
es rises rapidly with temperature because warmer molecules escape liquid or ice more easily. Around typical weather conditions, es increases by roughly 6–7% per °C near 20°C, which is why small temperature errors can strongly affect humidity calculations.
3) Over water versus over ice
Below 0°C, saturation differs depending on whether the reference surface is liquid water or ice. Over ice, es is lower because sublimation equilibrium is different. This matters for frost, snow surfaces, and cloud microphysics in cold air masses.
4) Common models used in practice
This calculator offers Magnus, Tetens, and Buck approximations. They are engineered curve-fits to laboratory data and are widely used in meteorology, HVAC, and environmental monitoring. Different constants produce slightly different results, especially outside standard temperature ranges.
5) Typical reference values
At 0°C over water, es is about 6.11 hPa (0.611 kPa). Near 20°C it is about 23.4 hPa (2.34 kPa), and by 30°C it is about 42.4 hPa (4.24 kPa). These checkpoints help validate instrument readings.
6) Linking to humidity and dew point
Relative humidity is often computed as RH = 100·e/es, where e is actual vapor pressure. Dew point is the temperature at which es equals the current e. Accurate es estimates improve both metrics.
7) Unit handling for reporting
Environmental data commonly uses hPa or kPa, while engineering workflows may use Pa, mmHg, or psi. The calculator converts internally to Pa, then reports in your selected unit. This prevents rounding drift when comparing datasets from different instruments.
8) Interpreting results responsibly
Approximations have intended validity ranges. For most near-surface meteorology, Magnus is a strong default; Buck is often preferred for careful work near freezing. If you input extreme temperatures, treat outputs as approximate and cross-check against a standard reference table.
FAQs
1) Is saturation vapor pressure the same as humidity?
No. It is a temperature-dependent limit. Humidity describes the actual water vapor present. Relative humidity compares actual vapor pressure to the saturation value at the same temperature.
2) When should I choose “over ice”?
Use “over ice” when the relevant surface is frozen, such as snowpack, frost, or ice crystals. It is also useful for cold-cloud calculations where equilibrium is closer to sublimation than evaporation.
3) Which model is best for everyday weather calculations?
Magnus is commonly used for routine meteorology near typical ambient temperatures. If you work near 0°C or need a more specialized fit, try Buck and compare differences.
4) Why do different formulas give slightly different answers?
They use different fitted constants and functional forms based on laboratory datasets and target ranges. Differences are usually small near room temperature but can grow at cold or very warm extremes.
5) How accurate are the results?
Within their recommended ranges, these approximations are typically close to reference values for operational use. Accuracy depends on temperature precision, phase choice, and whether conditions match the model’s assumptions.
6) Can I use this for dew point calculations?
Yes, indirectly. Dew point is found by solving for the temperature where saturation vapor pressure equals the actual vapor pressure. This calculator provides the saturation side needed for that workflow.
7) Why offer multiple output units?
Different disciplines report pressure differently. Meteorology often uses hPa, physics uses Pa, and some lab devices use mmHg or psi. Multiple units reduce conversion errors and improve reporting consistency.