Interactive psychrometric chart and moist air calculator
A psychrometric chart shows every property of moist air on one graph. Point at the chart, or enter a temperature and relative humidity, to read the rest. Change the altitude and the chart is recomputed for the pressure there.
Point at the chart to read a state, or click to pin it.
Saturated air at sea level
| T (°C) | p_s (kPa) | W_s (g/kg) | h_s (kJ/kg) |
|---|---|---|---|
| -10 | 0.2599 | 1.607 | -6.07 |
| -5 | 0.4017 | 2.487 | 1.17 |
| 0 | 0.6112 | 3.790 | 9.48 |
| 5 | 0.8726 | 5.425 | 18.65 |
| 10 | 1.2282 | 7.664 | 29.37 |
| 15 | 1.7057 | 10.695 | 42.14 |
| 20 | 2.3392 | 14.761 | 57.59 |
| 25 | 3.1697 | 20.172 | 76.54 |
| 30 | 4.2467 | 27.326 | 100.05 |
| 35 | 5.6286 | 36.744 | 129.50 |
| 40 | 7.3844 | 49.111 | 166.72 |
| 45 | 9.5944 | 65.353 | 214.19 |
| 50 | 12.3513 | 86.752 | 275.33 |
W = 0.621945 · p_w ÷ (p − p_w), h = 1.006·t + W·(2501 + 1.86·t)
How to read the chart
The horizontal axis is the dry-bulb temperature, what an ordinary thermometer reads. The vertical axis, on the right, is the humidity ratio: grams of water per kilogram of dry air. Every point below the top curve is a possible state of air.
The top curve is the saturation line, 100% relative humidity. The curves below it are 10% to 90% relative humidity. The slanted lines running down and to the right from the saturation line are lines of constant wet-bulb temperature, numbered where they start. The steep lines are constant specific volume.
Example
Air at 25 °C and 50% relative humidity has a humidity ratio of 9.93 g/kg, a wet bulb of 17.9 °C, a dew point of 13.9 °C, an enthalpy of 50.4 kJ/kg and a specific volume of 0.858 m³/kg. Cool that air and the point moves left along a horizontal line, because the amount of water does not change. Where it meets the saturation line, at 13.9 °C, water starts to condense: that is the dew point.
About the calculation
The chart is computed, not traced from a printed one. Saturation vapour pressure comes from IAPWS-IF97 over water and IAPWS R14-08 over ice, with the Buck enhancement factor, and the other relations from the ASHRAE equations. The saturated humidity ratio matches the ASHRAE tables to within 0.013% from −20 °C to 25 °C.
At altitude the pressure is lower, so the same relative humidity holds more water per kilogram of air. The code is open source on GitHub and npm as psychrometrics.