A mixture of dry air and vapor
Psychrometrics connects the temperature, moisture and energy of air. At a given pressure, dry-bulb temperature and one humidity property determine the state. The chart combines these relationships for a specific pressure: changing altitude changes the chart.
W is the mass of vapor per kilogram of dry air. This basis allows added or removed vapor to be tracked without changing the reference mass. Enthalpy h and specific volume v also use a kilogram of dry air. Density ρ counts the total mass of air and vapor per volume of mixture.
p and pw are total pressure and vapor partial pressure in the same units. W and q are mass ratios; multiplying by 1000 expresses them in g/kg.
Humidity quantities
| Quantity | Common names |
|---|
| W · g/kg dry air | Humidity ratio, mixing ratio / razón de humedad, razón de mezcla |
|---|
| q · g/kg moist air | Specific humidity / humedad específica |
|---|
| ρv · g/m³ | Absolute humidity, vapor density / humedad absoluta |
|---|
Absolute humidity is the mass of vapor per volume of mixture, ρv = W/v. W refers to the mass of dry air and q to the total mass of moist air. Each quantity expresses vapor content on a different basis.
Saturation, dew point and frost point
RH compares vapor pressure to saturation pressure at the same temperature. In this chart, it is expressed relative to liquid water, including below freezing.
Some charts and references use saturation over ice at low temperatures. The same RH percentage relative to water or ice then corresponds to different amounts of vapor. Comparing values requires the same reference.
Dew point Td satisfies pws,w(Td) = pw. Frost point uses saturation over ice, whose correlation covers −100 to 0.01 °C. It can be negative even when the air is above 0 °C. Dry air has no finite dew or frost point; values below −100 °C are marked outside the model domain.
At −10 °C, RHw = 100 % corresponds to about 110.2 % relative to ice. The air is supersaturated over ice, without predicting whether frost will form. Saturation pressures use Hyland–Wexler and, for supercooled water, Murphy–Koop; a continuous transition from 0 to 0.1 °C joins the two water correlations.
Wet bulb and energy
Thermodynamic wet-bulb temperature, Tw, is the saturation temperature reached through an adiabatic balance with water. A psychrometer wick exchanges heat and mass: its reading can approximate Tw, but depends on the instrument and ventilation.
The wet-bulb range in this chart covers liquid water and Tw ≥ 0 °C. If the balance has no solution in that interval, the remaining properties stay available. Wet bulb over ice is outside this scope. When all three temperatures are available, Td ≤ Tw ≤ T.
These expressions use T in °C, p in Pa, W in kg/kg, h in kJ/kg dry air and v in m³/kg dry air. In the equation for v, T + 273.15 is absolute temperature in kelvin (K). Chart temperatures can be expressed in °C or °F; changing the scale preserves the same air state.
The reference for h is dry air and liquid water at 0 °C. Negative values correspond to enthalpies below that reference.
How to read the psychrometric chart
Locate T on the horizontal axis and W on the right axis. Percentage curves indicate RH over water; the blue envelope is 100 %. The dashed violet line represents saturation over ice.
A horizontal projection from the point to water saturation gives dew point; the ice intersection gives frost point when available. The cyan line joins the point to saturation at the liquid-water wet bulb, when available. Enthalpy lines are oblique and labeled with their h value; the state value appears in Results. The wet-bulb line is nearly parallel to the enthalpy lines, although it represents a different quantity.
Pointer and keyboard inspection shows T, W, RH and h without changing inputs. Regions above water saturation are outside this gas-mixture chart. The view adapts to the state; a distant dew point can lie beyond the view.
Local pressure
With Z in meters, the standard atmosphere estimates p in Pa. At 18 °C and RH 70 %, W is about 9.00 g/kg at sea level and 12.49 g/kg at 2640 m, where p ≈ 73.38 kPa: an increase of roughly 39 %.
Determining W, h or v from temperature and RH also requires local pressure. Station pressure corresponds to the measurement location; pressure reduced to sea level includes an altitude correction. Converting pressure to altitude gives an equivalent altitude in the standard atmosphere.
Example air states
- 24 °C, RH 50 %, sea level: W = 9.299 g/kg, h = 47.81 kJ/kg and Td = 12.95 °C.
- Bogotá as an altitude example: 18 °C, RH 70 % and 2640 m give W = 12.49 g/kg. These are teaching inputs, not current observations.
- ASHRAE Example 1, T = 40 °C, Tw = 20 °C and p = 101.325 kPa: the ideal balance gives W = 6.401 g/kg, h = 56.72 kJ/kg and RH = 13.98 %. Reading the printed chart gives approximate values.
- Cold air: T = −10 °C and Td = −15 °C give RHw = 66.79 % and frost point −13.41 °C. Liquid-water wet bulb at ≥0 °C is outside scope.
Scope and precision
The calculation range is −40 to 60 °C and 50 to 110 kPa; the altitude range is −500 to 5500 m. It describes a gaseous state, without suspended droplets or processes between states. Vapor pressure deficit is evaluated at air temperature.
The ideal-gas model omits intermolecular interactions and the enhancement factor, which corrects vapor saturation pressure in the presence of air. Agreement with real-air properties depends on temperature, pressure and the property considered.
Results are displayed with four significant figures, expandable to ten. The additional figures show more of the calculated value; physical accuracy depends on the model approximations and the uncertainty of the inputs.
The LCL/CCL calculator estimates atmospheric condensation levels and uses the Bolton saturation correlation. That difference in correlation can produce dew-point values that differ from those in this chart.