Moist Air and the Psychrometric Chart
Humidity ratio, relative humidity, dew point, wet bulb and enthalpy, how to read the chart and how to calculate every property.
Moist air: dry air plus vapor
Air in a building is a mixture of dry air and water vapor. Both behave as ideal gases (see Ideal Gases and Processes), and by Dalton’s law the total pressure is the sum of their partial pressures: p = pa + pw. At a given temperature the air can hold vapor only up to the saturation pressure pws (the vapor pressure of water at that temperature: 3.17 kPa at 25 °C). Cooler air holds much less, which is why cold surfaces sweat.
The properties
| Property | Symbol and meaning | Formula (sea level, p = 101.325 kPa) |
|---|---|---|
| Dry-bulb temperature | T: what a thermometer reads | — |
| Humidity ratio | W: kg of vapor per kg of dry air | W = 0.622 pw ÷ (p − pw) |
| Relative humidity | φ: pw ÷ pws at the same T | φ = pw ÷ pws(T) |
| Dew point | Tdp: temperature at which the vapor starts to condense | pws(Tdp) = pw |
| Wet-bulb temperature | Twb: what a wetted thermometer reads in moving air | from an energy balance (see below) |
| Specific enthalpy | h: kJ per kg of dry air | h = 1.006 T + W (2501 + 1.86 T) |
| Specific volume | v: m³ per kg of dry air | v = 0.287 042 (T + 273.15)(1 + 1.6078 W) ÷ p |
The saturation pressure comes from the Hyland–Wexler formula (or a steam table, see Steam, Water and Property Tables). Two independent properties fix the state of the air, so any two (dry bulb and relative humidity, dry bulb and wet bulb, dry bulb and dew point) let you find all the others. The Psychrometric Calculator does it for you.
Worked example: 25 °C and 50 percent
Worked example: all the properties of room air
- Saturation pressure at 25 °C: pws = 3 169 Pa. At 50 percent, pw = 1 585 Pa.
- Humidity ratio W = 0.622 × 1 585 ÷ (101 325 − 1 585) = 9.88 g/kg.
- Enthalpy h = 1.006 × 25 + 0.00988 × (2 501 + 1.86 × 25) = 25.15 + 25.17 = 50.32 kJ/kg.
- Dew point: the temperature at which pws = 1 585 Pa is 13.86 °C. A window colder than this will fog.
- Wet-bulb temperature: 17.89 °C. Specific volume v = 0.8580 m³/kg (density of the dry air is about 1.17 kg/m³).
- A 300 m³ room holds 350 kg of dry air and 3.45 kg of water vapor.
Reading the chart
The psychrometric chart plots all of this at once. The horizontal axis is dry-bulb temperature, the vertical axis is humidity ratio, and the curved lines are relative humidity; the top-left curve is saturation (100 percent). Dew point is found by moving horizontally from the state (constant W) to the saturation curve; wet bulb by following a line of constant enthalpy to the saturation curve. Heating or cooling without moisture change moves a state horizontally, and humidifying moves it up (see Psychrometric Processes).
A table of states
| State | W (g/kg) | h (kJ/kg) | Dew point (°C) | Wet bulb (°C) | v (m³/kg) |
|---|---|---|---|---|---|
| 20 °C, 50 % | 7.26 | 38.6 | 9.3 | 13.8 | 0.840 |
| 25 °C, 50 % | 9.88 | 50.3 | 13.9 | 17.9 | 0.858 |
| 25 °C, 60 % | 11.90 | 55.5 | 16.7 | 19.5 | 0.861 |
| 30 °C, 70 % | 18.80 | 78.2 | 23.9 | 25.5 | 0.885 |
| 35 °C, 40 % | 14.13 | 71.5 | 19.4 | 23.9 | 0.893 |
| 0 °C, 80 % | 3.02 | 7.5 | -3.0 | -1.1 | 0.778 |
The first rows show how much more water warm air carries: 35 °C air at 40 percent holds about twice the water of 20 °C air at 50 percent. The last row, cold outdoor air, carries very little water however humid it feels.
Altitude, winter air and measurement
Worked example: three corrections
- Winter dryness. Outdoor air at −5 °C and 80 percent has W = 2.08 g/kg. Heated to 21 °C with no humidification the same W gives 14 percent relative humidity: far below the comfort range, which is why heated buildings in cold climates need humidifiers.
- Altitude. At 1 500 m the pressure is about 84.6 kPa, so the humidity ratio at 25 °C and 50 percent rises from 9.88 to 11.88 g/kg, and air is about 17 percent less dense (more airflow for the same mass flow). Use charts made for the altitude.
- A sling psychrometer reads 25 °C dry and 18 °C wet. The wet-bulb equation gives W = 10.02 g/kg, so the relative humidity is 51 percent. Keep the wick clean and the air moving at 3 to 5 m/s.
- Condensation check. With room air at 25 °C and 50 percent (dew point 13.9 °C) a glass pane at 10 °C will sweat; a pane above 13.9 °C will stay dry.
Related
- Previous: HVAC Basics and Thermal Comfort. Next: Psychrometric Processes.
- Psychrometric Calculator.
- Steam, Water and Property Tables.
- HVAC and Psychrometrics: the whole guide.
FAQ
Why does relative humidity change when I heat air?
Because the vapor pressure stays the same but the saturation pressure rises with temperature. The humidity ratio and dew point do not change when air is only heated or cooled.
What is the wet-bulb temperature for?
It tells you how far air can be cooled by evaporation, so it sets the limit of evaporative coolers and cooling towers.
Is a psychrometric chart accurate enough?
For most HVAC work, yes (a few percent). For process drying and critical humidity control calculate the properties.
What is grains per pound?
US charts show humidity ratio in grains of water per pound of dry air: 7 000 grains = 1 lb, so 1 g/kg = 7 grains/lb.
References
- ASHRAE, Handbook – Fundamentals, Chapter 1 (Psychrometrics); Hyland, R. W., Wexler, A., Formulations for the Thermodynamic Properties of the Saturated Phases of H₂O, ASHRAE Transactions 89 (1983).
- CIBSE Guide C, Reference Data; McQuiston, F. C., et al., Heating, Ventilating, and Air Conditioning, Wiley.
- Çengel, Y. A. and Boles, M. A., Thermodynamics: An Engineering Approach (gas–vapor mixtures).
Last reviewed: October 2026. Formulas follow ASHRAE at sea level unless stated.