Calculator Inputs
Formula Used
The calculator first estimates saturation vapor pressure. Then it converts that pressure into saturation mixing ratio and saturation specific humidity.
| Step | Formula | Meaning |
|---|---|---|
| Saturation vapor pressure | es = formula(T) | Estimated vapor pressure at saturation. |
| Mixing ratio | ws = εes / (p - es) | Mass of vapor per mass of dry air. |
| Specific humidity | qs = ws / (1 + ws) | Mass of vapor per mass of moist air. |
| Direct form | qs = εes / [p - (1 - ε)es] | Same value in one expression. |
Here, ε is 0.622. Pressure p and saturation vapor pressure es must use the same unit.
How to Use This Calculator
- Enter the air temperature.
- Select the matching temperature unit.
- Enter the total air pressure.
- Select the pressure unit.
- Choose a vapor pressure equation.
- Select water, ice, or automatic phase handling.
- Enter relative humidity to compare actual humidity with saturation.
- Press calculate and review the result above the form.
- Use the CSV or PDF button to save the result.
Example Data Table
| Temperature | Pressure | Formula | Phase | Approx. Saturation Specific Humidity |
|---|---|---|---|---|
| 0 °C | 1013.25 hPa | Bolton | Auto | 3.76 g/kg |
| 10 °C | 1013.25 hPa | Bolton | Water | 7.63 g/kg |
| 20 °C | 1013.25 hPa | Bolton | Water | 14.47 g/kg |
| 30 °C | 1013.25 hPa | Bolton | Water | 26.41 g/kg |
Guide to Saturation Specific Humidity
What It Means
Saturation specific humidity describes the largest water vapor share that air can hold at a chosen temperature and pressure. It is written as mass of vapor divided by mass of moist air. The value rises quickly when temperature increases, because warm air supports a higher saturation vapor pressure. The value falls when total air pressure increases, because the same vapor pressure becomes a smaller part of the air mixture.
Why This Tool Helps
This calculator is useful in weather work, HVAC review, classroom problems, agriculture, and climate checks. It accepts temperature, pressure, pressure units, phase selection, and formula choice. It also estimates actual specific humidity when relative humidity is entered. This helps compare saturated air with real air conditions. The graph shows how saturation changes across nearby temperatures while pressure stays fixed.
Specific Humidity and Relative Humidity
Specific humidity differs from relative humidity. Relative humidity is a percentage. It compares current vapor pressure with saturation vapor pressure. Specific humidity is a mass ratio. It stays easier to use in many conservation and energy calculations. Mixing ratio is also shown, because many meteorology tables use grams of vapor per kilogram of dry air.
Pressure Effects
Pressure matters. At high altitude, pressure is lower. Lower pressure can make the saturation specific humidity larger for the same temperature. That is why mountain air, aircraft cabins, and laboratory chambers need pressure-aware calculations. A sea level estimate may be wrong when pressure is far from standard pressure.
Formula Choice
Formula choice also matters. The Bolton, Tetens, Buck, and Sonntag equations are empirical approximations. They are very close for common weather ranges, but they may differ slightly near freezing or at high temperatures. The ice option is useful below zero degrees Celsius. The auto option switches phase based on temperature.
Practical Use
Use the output as an analytical estimate. Check instrument accuracy before using it for safety, medical, or industrial control. Enter realistic pressure values. Keep saturation vapor pressure below total pressure. Then compare kilograms per kilogram, grams per kilogram, and parts per thousand. These views make the same result useful for reports, spreadsheets, and charts. When preparing reports, include the selected equation, phase, pressure, and temperature unit. This makes the result transparent, repeatable, and easier to audit later during review.
FAQs
1. What is saturation specific humidity?
It is the maximum mass of water vapor in moist air at a given temperature and pressure. It is usually reported in kg/kg or g/kg.
2. Why does temperature affect the result?
Warmer air has higher saturation vapor pressure. This allows a larger water vapor share before condensation begins.
3. Why does pressure affect the result?
Total pressure changes the vapor share in the air mixture. Lower pressure usually raises saturation specific humidity for the same temperature.
4. Which formula should I use?
Bolton is a common weather choice. Buck and Sonntag are also strong options. Tetens is simple and widely used in teaching.
5. What does the ice option do?
The ice option uses an ice-surface vapor pressure relation. It is useful for temperatures below freezing.
6. Is specific humidity the same as relative humidity?
No. Relative humidity is a percentage. Specific humidity is a mass ratio of vapor to moist air.
7. What is saturation mixing ratio?
It is the mass of water vapor per mass of dry air at saturation. It is often shown in g/kg dry air.
8. Can I export my result?
Yes. Use the CSV button for spreadsheet work. Use the PDF button for simple reports and records.