A psychrometric chart is a graphical tool that maps the relationships between several properties of moist air, all on a single diagram. If you know just two properties of the air (say, its temperature and relative humidity), the chart lets you read off every other property: dew point, moisture content, energy content, and more. Engineers use it constantly for heating, cooling, and ventilation design, but the chart is also useful in agriculture, meteorology, and any field where controlling air moisture matters.
What the Chart Actually Shows
At its core, a psychrometric chart describes air at a constant atmospheric pressure. The horizontal axis is dry-bulb temperature, which is simply the air temperature you’d measure with an ordinary thermometer. The vertical axis on the right side is the humidity ratio, sometimes called moisture content, expressed as the weight of water vapor per unit weight of dry air. These two axes create the framework, but the chart packs in much more information through overlapping lines and curves.
The most prominent feature is a curved boundary sweeping from the lower left to the upper right. This is the saturation curve, representing air that holds 100 percent of the moisture it can at a given temperature. Any point on that curve means the air is fully saturated. Any point below and to the right of it represents air that could still absorb more moisture. You cannot plot a real-world condition above or outside that curve, because air physically cannot hold more water vapor than its saturation limit at a given temperature.
The Key Properties on the Chart
Six or seven properties are layered onto the same diagram, each represented by a different set of lines. Once you locate a single “state point” where two known values intersect, every other property can be read directly.
- Dry-bulb temperature: Shown as vertical lines running from bottom to top. This is the standard air temperature.
- Wet-bulb temperature: Shown as diagonal lines running from the upper left down toward the lower right. Wet-bulb temperature is what a thermometer reads when its sensor is wrapped in a damp wick and exposed to airflow. Evaporation cools the wick, so the wet-bulb reading is always lower than the dry-bulb reading, except when relative humidity reaches 100 percent, at which point the two are equal.
- Dew point temperature: Read by following a horizontal line from your state point leftward until it hits the saturation curve. The temperature at that intersection is the dew point: the temperature at which the air would start forming condensation.
- Relative humidity: Shown as a family of curves sweeping across the chart from left to right. The saturation curve itself is the 100 percent line. Curves below it represent 90, 80, 70 percent, and so on, down to very dry air near the bottom.
- Humidity ratio: Read on the vertical scale at the right edge. It tells you the actual mass of water vapor in the air, independent of temperature.
- Enthalpy: Represented by diagonal lines (often along the same slope as wet-bulb lines, with a separate scale along the saturation curve). Enthalpy captures the total energy content of the air, combining both the heat you can feel (sensible heat) and the energy stored in the water vapor (latent heat).
- Specific volume: Another set of diagonal lines, spaced wider apart, showing how much space a unit mass of the air mixture occupies. Warmer, moister air takes up more volume.
A useful detail about the saturation curve: it carries three temperature scales simultaneously. At 100 percent humidity, the dry-bulb, wet-bulb, and dew point temperatures all converge to the same value. So any point on that upper boundary can be read as any of the three.
How To Find a State Point
Reading the chart starts with two known values. Suppose you measure the air at 80°F dry-bulb and 50 percent relative humidity. Find 80°F on the horizontal axis and draw a vertical line upward. Then find the 50 percent relative humidity curve and follow it until it crosses your vertical line. That intersection is your state point.
From there, follow a horizontal line to the right edge to read the humidity ratio. Follow it horizontally to the left until it touches the saturation curve to find the dew point. Follow the nearest diagonal wet-bulb line up to the saturation curve to read wet-bulb temperature. Each property is embedded in the geometry of the chart, so no calculations are needed once you can identify the correct line families.
Why HVAC Engineers Rely on It
The chart’s real power shows up when you need to change the air from one condition to another. Heating, cooling, humidifying, and dehumidifying all trace specific paths across the chart, and those paths reveal exactly how much energy is required.
Pure heating (running air over a hot coil without adding or removing moisture) moves the state point horizontally to the right. The humidity ratio stays constant, but relative humidity drops because warmer air can hold more moisture. Pure cooling moves the point to the left, increasing relative humidity. If you cool air below its dew point, moisture condenses out, and the state point slides along the saturation curve, simultaneously lowering both temperature and moisture content. This is how air conditioning systems dehumidify: they chill the air past the dew point, wring out water, then sometimes reheat the air to a comfortable temperature.
Mixing two air streams, a routine task when outdoor air is brought in for ventilation, plots as a straight line between the two state points on the chart. The exact location of the mixed-air point depends on the proportion of each stream. Engineers can read the resulting temperature, humidity, and energy content directly, which determines how much additional heating or cooling the system needs.
Beyond basic comfort cooling, the chart supports design for cooling towers, indoor swimming pools, clean rooms, and evaporative cooling systems. Energy recovery devices, economizer cycles that use cool outdoor air to reduce compressor work, and supply-air temperature reset strategies can all be plotted and analyzed on the same diagram.
The Role of Atmospheric Pressure
Standard psychrometric charts are printed for sea-level atmospheric pressure (14.696 psi or 101.325 kPa). All the property relationships shift when pressure changes, which means a chart made for sea level is not accurate at high altitude. A city at 5,000 feet has noticeably lower atmospheric pressure, and air at that elevation holds moisture differently. Engineers working in Denver or Mexico City need a chart corrected for local altitude, or they need software that accounts for the pressure difference automatically. Using a sea-level chart in a high-altitude location will produce errors in humidity ratio, enthalpy, and equipment sizing.
Digital Tools and Software
Paper psychrometric charts are still printed and widely taught, but most professional work now happens in software. ASHRAE, the organization that publishes the standards behind most building climate control, offers a dedicated psychrometric chart app for iOS that lets engineers plot processes interactively, toggle between imperial and metric units, and email the results as a PDF. Their desktop program, Psychrometric Analysis (currently at version 9.0), handles full process analysis with energy calculations displayed graphically.
For engineers who work in spreadsheets or calculation environments, ASHRAE also provides add-in libraries for Excel, MATLAB, Mathcad, and EES, built on an updated numerical model for real moist air. These tools perform the same lookups a paper chart does, but with more decimal places and the ability to adjust for any altitude or pressure automatically. The underlying science is identical. The chart is simply a visual interface for a set of thermodynamic equations, and software replaces the ruler-and-pencil approach with faster, more precise computation.
A Brief Origin Story
The psychrometric chart as a formal engineering tool traces back to Willis Carrier, widely regarded as the father of modern air conditioning. In 1911, Carrier presented a paper titled “Rational Psychrometric Formulae” to the American Society of Mechanical Engineers, laying out the mathematical relationships that underpin the chart. That paper gave engineers a systematic, visual way to work with moist air properties, and its basic structure has remained remarkably stable for over a century. The axes, the saturation curve, and the overlapping property lines on a modern chart would all be recognizable to someone reading Carrier’s original work.