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Dry-bulb temperature, wet-bulb temperature and enthalpy describe different properties of moist air. Dry-bulb is the ordinary air-temperature reading; wet-bulb indicates how evaporation cools a wetted thermometer under specified conditions; and enthalpy describes the air’s energy content. On a psychrometric chart, dry-bulb and thermodynamic wet-bulb temperatures can locate a moist-air state when the chart’s pressure basis is known. From that state, you can determine enthalpy and other properties. For building operators and HVAC learners, keeping these three ideas distinct helps explain why air at the same dry-bulb temperature can behave differently when its moisture content or pressure differs.
What is the difference between dry-bulb and wet-bulb temperature?
Dry-bulb temperature is the ordinary temperature of the air, measured with a thermometer shielded from solar and long-wave radiation. Wet-bulb temperature describes the temperature associated with evaporation from a wetted thermometer bulb. The first tells you the air’s temperature without directly expressing its water-vapor content; the second reflects both temperature and moisture through evaporative cooling.
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ASHRAE Terminology defines dry-bulb temperature as the “temperature of air indicated by an ordinary thermometer shielded from solar and long wave radiation.” That shielding matters: radiation can affect a thermometer’s reading, so an unshielded instrument exposed to sunlight or nearby warm surfaces may not indicate the air temperature alone. The dry-bulb value is still only one description of the air state, not a measure of how much moisture the air contains.
Wet-bulb temperature is not simply another name for dry-bulb temperature, nor is it a direct reading of relative humidity. In the thermodynamic definition, it is the temperature at which evaporation brings moist air to saturation adiabatically at constant total pressure. The evaporating water draws heat from the air, lowering the wetted surface’s temperature. How far the wet-bulb temperature falls below the dry-bulb temperature depends on the air’s moisture as well as its temperature.
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A psychrometer’s measured wet-bulb reading approximates the thermodynamic wet-bulb temperature; the two are not guaranteed to be identical. Heat exchange with surrounding surfaces, the temperature of the water supplied to the wick and airflow over the wick can all affect the measurement. ASHRAE describes wet-bulb temperature as approximately equal to adiabatic saturation temperature for mixtures of water vapor and dry air at atmospheric temperatures and pressures. The word “approximately” is important: the definition describes an idealized process, while a physical instrument operates in real conditions. See the ASHRAE Handbook chapter on psychrometrics for the property definitions and their use.
What does wet-bulb temperature tell you about humidity?
Wet-bulb temperature gives a moisture-sensitive temperature measure, but it is not itself a humidity percentage. It can help distinguish two air states with the same dry-bulb temperature: the state with more moisture generally has less capacity for further evaporation under comparable conditions, so its wet-bulb temperature is closer to its dry-bulb temperature. To quantify moisture, use a property such as relative humidity or humidity ratio rather than treating the wet-bulb reading as either one.
Relative humidity and humidity ratio are distinct. Relative humidity describes moisture in relation to the saturation condition at the air’s temperature; humidity ratio is the mass of water vapor relative to the mass of dry air. Their relationship changes with temperature, so a relative-humidity percentage alone does not state a fixed amount of water vapor independent of the air temperature. A psychrometric chart can show both properties for a known state, but they answer different questions.
In practical building work, dry-bulb temperature is useful when the question is simply how warm or cool the air is. Wet-bulb temperature becomes useful when evaporative behavior or moisture conditions matter. Neither alone gives the whole state. For example, knowing only that a room’s air is at a particular dry-bulb temperature does not establish its humidity ratio, enthalpy or dew point. You need another measured or derived property, along with the relevant pressure basis, to establish those values.
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What is enthalpy in moist air?
Enthalpy is an energy property, not another thermometer reading. In ASHRAE’s perfect-gas treatment, moist-air enthalpy is the sum of the enthalpy contributions from dry air and water vapor. It is commonly reported as specific enthalpy per unit mass of dry air, for example in kJ/kg dry air. Keeping the dry-air mass basis attached to the units prevents confusion about what the reported quantity represents.
That combined energy description matters in HVAC because an air process can change temperature, moisture content, or both. A dry-bulb reading can show a temperature change but does not, by itself, capture the energy associated with a change in water vapor. Enthalpy offers a way to describe the energy content of the moist-air mixture at a state. It is therefore useful alongside temperature and moisture properties when analyzing heating, cooling, humidification and dehumidification.
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Enthalpy is not interchangeable with dry-bulb temperature, wet-bulb temperature, relative humidity or humidity ratio. Each describes something different: temperature, evaporative response, moisture relative to saturation, moisture mass relative to dry-air mass, or energy content. A psychrometric chart brings those properties together for a specified pressure so you can relate them without pretending they are the same measurement.
How do I read enthalpy on a psychrometric chart?
First establish the pressure basis and the air state. For a specified pressure, dry-bulb temperature and thermodynamic wet-bulb temperature can identify a state point; the chart can then be used to read enthalpy and other moist-air properties. Do not read a value from a chart until you know which pressure conditions it represents.
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- Check the chart’s pressure basis. Confirm that it corresponds to the barometric pressure relevant to the location or analysis. A sea-level chart is not automatically suitable for an elevated site.
- Find the dry-bulb temperature. Use the chart’s dry-bulb scale to identify the air temperature you have measured or specified.
- Find the thermodynamic wet-bulb temperature. Locate the wet-bulb value on the chart. If your input is a physical psychrometer reading, remember it is an approximation to the thermodynamic value and can be affected by measurement conditions.
- Locate the state point. The intersection of the dry-bulb and thermodynamic wet-bulb conditions identifies the air state for that pressure.
- Read the property you need. At that state point, use the chart to determine enthalpy, humidity ratio, dew-point temperature, relative humidity or specific volume. Follow the chart’s own lines and units, and record the pressure basis with derived values.
These steps describe the logic of the chart rather than a particular chart layout. Read its legend carefully: scale placement, units and pressure assumptions can differ. If you calculate rather than read a chart, use equations consistent with the same pressure and unit basis.
ASHRAE’s SI handbook gives a worked example at 40°C dry-bulb, 20°C thermodynamic wet-bulb and 101.325 kPa. For that specific state and pressure, the example reports a humidity ratio of 6.5 g water/kg dry air, enthalpy of 56.7 kJ/kg dry air, dew point of 7°C, relative humidity of 14%, and specific volume of 0.896 m³/kg dry air. These are values from the handbook’s example, not universal values for any air at 40°C or a general rule about how those properties relate. Change the wet-bulb condition or pressure and the derived properties change.
Why does pressure matter when using a psychrometric chart?
Moist-air properties derived from a dry-bulb and wet-bulb pair depend on pressure. The pressure basis is not a minor chart detail: the same temperature pair can correspond to different humidity ratios, enthalpies and specific volumes at different barometric pressures. Use the chart for the relevant pressure or an appropriate calculation; do not assume a sea-level chart and an altitude chart are interchangeable.
ASHRAE’s comparison of sea-level and 1500 m conditions reports that, for a given dry-bulb and wet-bulb pair, humidity ratio and enthalpy increase with altitude while relative humidity changes little. Specific volume changes substantially with barometric pressure. This comparison illustrates why a chart’s pressure basis must accompany any values read from it. It is not a universal conversion rule that lets you correct every chart value by a single factor.
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For a building operator, this caveat applies whenever chart-derived properties are recorded, compared or used in a process calculation. Note the location or stated pressure basis, the units and the input conditions. If a chart or calculation produces enthalpy in kJ/kg dry air, retain that basis in reports and comparisons. An enthalpy number detached from its units and air-state assumptions is easy to misread.
How are the three properties useful in HVAC and building operation?
Use the property that answers the process question, and use more than one when the process changes both temperature and moisture. Psychrometric charts can represent heating, cooling, humidification and dehumidification. These processes are easier to understand when you track dry-bulb temperature, wet-bulb temperature, enthalpy, humidity ratio and pressure as related but separate properties.
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- Cooling: A dry-bulb change describes cooling in temperature terms. Enthalpy and moisture properties help characterize the air state more completely when water vapor is relevant to the process.
- Humidification: Since moisture is being added, dry-bulb temperature alone cannot describe the resulting state. Use a chart or calculation at the correct pressure to determine the other properties.
- Dehumidification: Since moisture content is changing, distinguish humidity ratio from relative humidity and consider enthalpy alongside temperature when the process question concerns energy as well as moisture.
This is a framework for selecting properties, not a substitute for equipment-specific design calculations. The useful comparison depends on what is being evaluated: a temperature condition, a moisture change, an energy change, or a combination. The chart’s role is to show how the properties relate at a defined pressure, not to make a process decision without its operating context.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What are the common mistakes to avoid?
- Calling dry-bulb temperature a measure of moisture. It is the ordinary air-temperature reading. Pair it with another property if moisture content matters.
- Treating wet-bulb as a humidity percentage. It responds to moisture through evaporation, but it is a temperature and is not the same as relative humidity or humidity ratio.
- Equating every measured wet-bulb value with the thermodynamic value. A wetted-wick instrument is affected by its conditions; use the reading as an approximation, not an identity.
- Using a chart without checking pressure. Pressure changes chart-derived properties. Identify the chart basis before recording or applying values.
- Dropping the dry-air basis from enthalpy units. Report enthalpy in the stated units per unit mass of dry air, such as kJ/kg dry air.
- Confusing relative humidity with humidity ratio. They are different properties, and their relationship changes with temperature. Use the one that answers the question being asked.
For further study, ASHRAE describes a psychrometrics course covering chart theory, moist-air properties, HVAC processes and airflow calculations in its Summary of Psychrometrics course listing. Course availability may change.
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Frequently Asked Questions
Can dry-bulb temperature alone tell me whether a room is humid?
No. Dry-bulb temperature records air temperature but does not directly state the amount of water vapor. To describe moisture, you also need a moisture-related property, such as relative humidity or humidity ratio, interpreted with the air temperature and pressure.
Why is the wet-bulb reading lower than the dry-bulb reading?
Evaporation from the wetted bulb draws heat from it, lowering its temperature. How much cooling occurs depends on the air’s moisture and the measurement conditions; the difference is not a standalone humidity percentage.
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What units should I use for moist-air enthalpy?
Use the units specified by the chart, equation or engineering convention, and preserve the mass basis. In the cited ASHRAE SI treatment, specific enthalpy is expressed per unit mass of dry air, such as kJ/kg dry air.
Does an altitude change mean a building’s relative humidity must change?
Not necessarily. ASHRAE’s cited comparison finds relative humidity changes little for its specified same dry-bulb/wet-bulb pair between sea level and 1500 m, while other derived properties change. That comparison should not be treated as a prediction for every real building or changing air state.
