Physics of Buildings

Physics of Buildings · Air

Psychrometric chart

Each point is one parcel of air. Drag it and read every property at once, or learn to read the chart one family of lines at a time.

Drag the red point, or type its dry bulb and relative humidity. Process mode adds a second point B and the move from A to B (try the presets); Mix blends two streams. Altitude moves the saturation curve; changing it keeps your dry bulb and RH and recomputes the rest. Any A-to-B arrow is bookkeeping between two states, not necessarily a path one device takes. Computed from published equations, not read off a printed chart.

A psychrometric chart is a map of one parcel of air. Across the bottom is its temperature; up the side is how much water vapour it carries. Pick any two properties and you have a point, and from that point every other property can be read off along a different family of lines [1]. Heating, cooling, drying, humidifying and mixing are all moves on the map, and condensation is what happens when a move to the left runs into the curved edge.

What do you want to read?

How to read the chart, one family of lines at a time

The chart below is the same one as at the top, pinned while you read. Each step adds one family of lines and reads one property from the same parcel of air: 25 °C at 50 % relative humidity, at sea level. Change anything to explore on your own; Return to the story hands control back.

Figure 1 The same chart, pinned while you read. Each step shows one family of lines; drag the point or change anything to take over.#

1. Dry bulb: straight down

The point is one parcel of air at 25 °C. Vertical lines are lines of constant dry-bulb temperature, the reading of an ordinary thermometer, and you read it straight down on the bottom scale [1]. Every point on a vertical line is equally warm; what changes as you go up it is the water.

2. Humidity ratio: straight across

Horizontal lines are lines of constant humidity ratio: the mass of water vapour carried by each kilogram of dry air. Read it across on the right-hand scale. This parcel carries 9.9 grams of water per kilogram of dry air (69 grains per pound). Because it counts water against dry air, it does not change when the air warms or cools; only adding or removing water moves the point up or down.

Question 1 Prediction: where does the walk to the left end? Answered by step 3.#

3. Dew point: walk left to the curve

The curved upper edge is the saturation curve: the most water air can carry at each temperature. Cooling without changing the water is a walk to the left along the horizontal line. Where the walk meets the curve, the air is saturated, and the temperature there, read straight down, is the dew point: 13.9 °C for this parcel [2][3]. Drag the point left past it and the point slides down the curve instead, because any surface colder than 13.9 °C pulls water out of this air as dew. Below 0 °C the saturation curve is over ice, and the same point is called the frost point.

4. Relative humidity: the curves

The blue curves are lines of constant relative humidity: the vapour pressure in the air as a share of the vapour pressure of saturated air at the same dry bulb [2]. The saturation curve is 100 %; the bottom axis is 0 %. This parcel sits on the 50 % curve. Notice how the curves fan out: at 25 °C each 10 % of RH is about 2 g/kg of water, at 5 °C about 0.5 g/kg. RH says how close the air is to saturation, not how much water it holds.

5. Wet bulb: up the slanted lines

The dashed lines slope up to the left. Each is a line of constant wet-bulb temperature: the temperature a wet wick settles at as water evaporates from it into this air, the lowest the air can be cooled by evaporating water into it [2]. Follow the line through the point up to the saturation curve and read the temperature there: 17.9 °C. The wet bulb always sits between the dew point and the dry bulb, and all three are equal at saturation.

6. Enthalpy: the energy scale

The amber lines are lines of constant enthalpy, the heat content of the air per kilogram of dry air, counting both its warmth and the latent heat locked in its vapour [4]. Their labels sit just beyond the saturation curve. This parcel holds 50.3 kJ/kg (on the IP chart 29.3 Btu/lb; the two scales start from different zeros). Enthalpy lines run almost parallel to wet-bulb lines, which is why an evaporative cooler, which adds no heat, moves air along a wet-bulb line.

7. Specific volume: the steep lines

The steep grey lines give the specific volume: the cubic metres of moist air that contain one kilogram of dry air. This parcel takes up 0.858 m³ per kilogram of dry air. It matters when a fan, which moves volume, has to deliver a mass of air: warm, humid air takes more room per kilogram. That is the whole chart: two properties fix the point, and every other one is a line through it.

Question 2 Prediction: what heating does to relative humidity. Answered by step 8.#

8. Heating: a move to the right

Now the chart shows a process, a move from A to B. Heating with a furnace coil or a heat pump adds no water, so the point moves straight to the right. Air at 0 °C and 80 % RH carries 3.0 g/kg; at 21 °C that same water is only 20 % RH. The frost point (the dew point over ice) stays at −2.7 °C. Adding the warmth took 21.2 kJ per kilogram of dry air, all of it sensible (temperature) heat. This is why heated winter air feels dry: nothing was taken out of it.

9. A cooling coil: down and to the left

An air-conditioning coil is colder than the dew point of the air crossing it. Air that touches the fins is chilled to near the coil surface and gives up water; air that slips between the fins is cooled less. The leaving air lands on a straight line from the entering state toward the coil's apparatus dew point, here 10 °C, about 15 % of the way back [5]. From 27 °C and 50 % RH the air leaves at 12.6 °C and 90 % RH, and 3.0 g of water per kilogram of dry air has run off the coil into the drain pan. Of the 22.4 kJ/kg removed, two-thirds is sensible heat and one-third latent (the water).

Air-conditioning coils sit on refrigerant circuits. This explains the air side only; work on the refrigerant side is for a licensed HVAC technician.

10. A cold surface: walk left into the curve

Room air at 21 °C and 50 % RH has a dew point of 10.2 °C. Let it touch a surface at 5 °C, a single-pane window in winter or a cold-water pipe, and the air next to it walks left, reaches the saturation curve at 10.2 °C and then slides down it. At 5 °C it can carry only 5.4 g/kg, so 2.3 g/kg condenses on the surface [3]. Condensation in buildings is always this move. The essay on dew point and condensation follows it into windows, walls and ducts.

11. Evaporative cooling and humidifying: up the wet-bulb line

Blow hot, dry air through a wet pad and water evaporates into it. The heat to evaporate it comes out of the air itself, so the point moves up and to the left along a wet-bulb line, with almost no change in enthalpy. No pad reaches the wet bulb itself (17.4 °C for desert air at 35 °C and 15 % RH): Nebraska Extension puts the air leaving cooling pads 3 to 5 °F above it [1]. The preset stops 4.7 °F short, at 20.1 °C and 77 % RH, after the air has gained 6.2 g of water per kilogram. The same move, run in a dry winter house, is a wetted-media humidifier.

12. Mixing two streams: a straight line

Mixing outdoor air with return air puts the mixture on the straight line between the two points, at the share of each stream measured by dry-air mass: water and energy are both conserved, so both average [4]. With 30 % outdoor air at −5 °C and 80 % RH and 70 % return air at 22 °C and 40 % RH, the mix is 14 °C and 53 % RH. Drag A to cold, damp air and B to warm, humid air: when the straight line between them cuts across the curved saturation line, the mix is supersaturated and the excess water appears as fog, the same thing as your breath on a winter morning [5].

13. Altitude changes the chart

Every line above depends on the barometric pressure. At 1600 m (Denver is about that high) the standard atmosphere's pressure is 83.5 kPa instead of 101.3 [6]. Water vapour then makes up a bigger share of each kilogram of air: saturated air at 25 °C carries 24.5 g/kg instead of 20.1, so the saturation curve sits higher. The dew point of 25 °C, 50 % RH air is unchanged at 13.9 °C, but its humidity ratio is 12.0 g/kg instead of 9.9. Charts printed for sea level misread mountain air; set the altitude here instead.

Every process is a move

Once the point is fixed, the processes of heating, ventilating and air conditioning become directions on the map. Straight right is sensible heating; straight left is sensible cooling, until the saturation curve stops it. Straight up adds water (a steam humidifier adds it with almost no change in temperature); straight down removes it at constant temperature, which no single device does: a cold coil moves air down and to the left, cooling it as it dries it (so straight down takes a coil plus reheat), and a desiccant moves air down and to the right, because the heat released as it takes up water warms the air [5]. Up and to the left along a wet-bulb line is evaporative cooling. Down and to the left is the cooling coil. A mixture lies on the straight line between its streams.

The process mode in the chart at the top does the bookkeeping for any move you draw. It splits the change in enthalpy into sensible heat (the change at constant water content) and latent heat (the change in water content times the latent heat of the vapour), and reports the water added or removed per kilogram of dry air. The ratio of sensible to total, the sensible heat ratio, is the slope designers use to choose a coil: a ratio of 1 is pure temperature change, a lower ratio means more of the coil's work goes into drying the air.

The comfort zone, and what this chart leaves out

Many printed charts carry a shaded comfort zone. The usual one comes from ASHRAE Standard 55, a copyrighted standard, and it is not one fixed shape: it is computed from six inputs, the air temperature, the mean radiant temperature of the surrounding surfaces, the air speed, the humidity, the occupants' metabolic rate and their clothing [7]. Change the clothing or the radiant temperature and the zone moves. For that reason this chart does not draw one. The CBE Thermal Comfort Tool from UC Berkeley computes the Standard 55 zone for your own inputs and draws it on a psychrometric chart [7].

What the chart can show is a humidity target that has a public source. The U.S. EPA advises keeping indoor relative humidity below 60 percent, ideally between 30 and 50 percent, to limit mold [8]. Tick “EPA indoor humidity guidance” under Lines on the chart to shade that band. It is a moisture guideline, not a comfort zone.

Where the numbers come from

Nothing on this chart is traced from a printed one. Every line and readout is computed in your browser from published equations: the Hyland–Wexler formulas for the saturation pressure of water and ice, valid from −100 to 200 °C, and the ideal-gas relations for humidity ratio, enthalpy, specific volume and the thermodynamic wet bulb, as set out in the ASHRAE Handbook—Fundamentals and implemented in the open-source PsychroLib library [4][9]. Pressure at altitude follows the U.S. Standard Atmosphere [6].

The saturation pressures agree with the IAPWS-95 values published by NIST to better than 0.05 % from 0 to 100 °C [10], and with the IAPWS sublimation formula over ice to the same tolerance from −100 to 0 °C [11]. The chart itself spans −10 to 50 °C (14 to 122 °F) and 0 to 30 g/kg; the calculator covers −40 to 60 °C.

A century-old map

Willis Carrier drew the first psychrometric chart in 1904; the chart's century of revisions since then is traced by Donald Gatley in the ASHRAE Journal [13]. The modern ASHRAE chart, with humidity ratio up the side and enthalpy lines across it, is the descendant used by every HVAC designer. The axes have stayed because they make the processes straight lines: heating is horizontal, adding water is vertical, and mixing is a straight chord.

Go deeper

The essay Why windows, walls and pipes sweat follows the walk to the left into buildings: winter windows, summer ducts, cold pipes and wall cavities, with a window you can chill and a room whose humidity you can raise. The psychrometric calculator solves a state from dry bulb plus relative humidity, wet bulb, dew point or humidity ratio, and says in words when an input is impossible.

Physics of Buildings explains the systems inside homes and commercial buildings through their physics. Every model is checked by an independent verifier against the sources cited; see about this site and the sources. The first piece in the series explains why a well pump keeps cycling.

Sources

  1. Shelton, D. P. (2008). Air Properties: Temperature and Relative Humidity, NebGuide G1849. University of Nebraska–Lincoln Extension. university extension Dry-bulb lines vertical, wet-bulb lines sloping, dew-point lines horizontal, RH curves sweep from lower left to upper right; any two properties fix the rest. Example 1: 80 °F and 50 % RH → wet bulb 67 °F, dew point 59 °F (read from a simplified chart).
  2. Bucklin, R. A., Haman, D. Z., Bayabil, H. K. & Guzman, S. M. (2019). Using Psychrometric Chart for Frost Protection, ABE 372. University of Florida IFAS Extension (EDIS). university extension Definitions: wet bulb is the lowest temperature to which air can be cooled by evaporating water into it; dew point is where moisture starts to condense when air is cooled at constant pressure and humidity ratio; relative humidity is the ratio of the actual vapour pressure to the saturation vapour pressure at the same dry bulb.
  3. U.S. Environmental Protection Agency, Indoor Environments Division (2013). Moisture Control Guidance for Building Design, Construction and Maintenance, EPA 402-F-13053. U.S. EPA, Washington, DC. government guidance pp. 14–17: dew point vs relative humidity; 85 °F at 60 % RH has a dew point of 70 °F (read along the chart to the saturation curve); at 90 % RH a surface 3 °F colder than the air condenses, at 50 % about 20 °F; a 55 °F duct sweats in air with a 70 °F dew point; ventilation dries a building only when the outdoor dew point is below the indoor one; windows are usually the first cold-weather condensation sites; thermal bridges and air leaks chill indoor surfaces.
  4. Meyer, D. & Thevenard, D. (2019). PsychroLib: a library of psychrometric functions to calculate thermodynamic properties of air. Journal of Open Source Software 4(33), 1137. doi:10.21105/joss.01137. peer-reviewed software paper Open-source implementation (C, Fortran, JavaScript, Python, VBA) of the moist-air equations of the ASHRAE Handbook—Fundamentals (2017), chapter 1.
  5. Indian Institute of Technology Kharagpur (NPTEL) (2008). Refrigeration and Air Conditioning, Lesson 28: Psychrometric processes (Version 1 ME). National Programme on Technology Enhanced Learning, India. university course text §28.2(b): apparatus dew point is the effective coil surface temperature; by-pass factor BPF = (T_C − T_S)/(T_O − T_S), leaving state on the cooling and dehumidification line toward the ADP (Fig. 28.3). §28.2(g)(ii): when very cold air mixes with warm humid air the mixture may fall in the two-phase region; water condenses, the humidity ratio falls and the temperature rises from the released latent heat, the phenomenon that forms fog. Worked problems 8–10 give reference states.
  6. NOAA, NASA and U.S. Air Force (1976). U.S. Standard Atmosphere, 1976 (NOAA-S/T-76-1562, NASA-TM-X-74335). U.S. Government Printing Office, Washington, DC. government standard Sea-level pressure 101 325 Pa, temperature 288.15 K, tropospheric lapse rate 6.5 K/km; the pressure–altitude relation behind ASHRAE eq 3.
  7. Tartarini, F., Schiavon, S., Cheung, T. & Hoyt, T. (2020). CBE Thermal Comfort Tool: online tool for thermal comfort calculations and visualizations. SoftwareX 12, 100563 (tool at comfort.cbe.berkeley.edu). doi:10.1016/j.softx.2020.100563. peer-reviewed software paper Implements ASHRAE Standard 55; inputs are air temperature, mean radiant temperature, air speed, relative humidity, metabolic rate and clothing insulation; the comfort zone drawn on its psychrometric chart depends on the mean radiant temperature chosen.
  8. U.S. Environmental Protection Agency (2026). A Brief Guide to Mold, Moisture and Your Home. U.S. EPA (web page, updated 18 February 2026). government guidance “If possible, keep indoor humidity below 60 percent (ideally between 30 and 50 percent) relative humidity.” Condensation on windows, walls or pipes can be a sign of high humidity.
  9. Meyer, D. & Thevenard, D. (2019). PsychroLib source code, JavaScript implementation (src/js/psychrolib.js). GitHub, psychrometrics/psychrolib (MIT licence). source code Comments give the ASHRAE (2017) ch. 1 equation numbers: saturation pressure eqs 5 and 6 (Hyland–Wexler; ice at or below the 0.01 °C triple point; valid −100 to 200 °C); humidity ratio eq 20 (0.621945·pw/(p − pw)); specific volume eq 26 (R_da = 287.042 J/kg·K, factor 1.607858); enthalpy eq 30 (1.006·t + W·(2501 + 1.86·t) kJ/kg; IP 0.240·t + W·(1061 + 0.444·t) Btu/lb); wet bulb eqs 33 and 35; standard atmosphere eq 3 (101 325·(1 − 2.25577×10⁻⁵·Z)^5.2559 Pa).
  10. Lemmon, E. W., McLinden, M. O. & Friend, D. G. (NIST Thermophysical Properties of Fluid Systems) (2025). Saturation Properties for Water: Temperature Increments, NIST Chemistry WebBook, SRD 69 (IAPWS-95 formulation of Wagner & Pruss 2002). National Institute of Standards and Technology. reference data Saturation pressure of water: 0.611654 kPa at 0.01 °C, 1.22902 at 10.01, 2.34077 at 20.01, 4.24941 at 30.01, 7.38888 at 40.01, 12.3581 at 50.01, 101.454 kPa at 100.01 °C.
  11. International Association for the Properties of Water and Steam (2011). Revised Release on the Pressure along the Melting and Sublimation Curves of Ordinary Water Substance, IAPWS R14-08(2011). IAPWS. standards body release Sublimation pressure of ice Ih, eq (6): ln(p/pt) = (Tt/T)·Σ ai·θ^bi with pt = 611.657 Pa, Tt = 273.16 K; valid 50 K to 273.16 K. Used here only as an independent check of the ice branch.
  12. World Meteorological Organization, Commission for Instruments and Methods of Observation (2018). Guide to Instruments and Methods of Observation (WMO-No. 8), Volume I, Chapter 4: Measurement of humidity (2018 edition). World Meteorological Organization, Geneva. standards body guide Annex 4.B eq 4.B.5: enhancement factor of moist air f(p) = 1.0016 + 3.15×10⁻⁶·p − 0.074/p (p in hPa), 1.0047 at 1013 hPa; its pressure dependence 0 to +0.6 %. Section 4.3: psychrometers need ventilation of 3 to 5 m/s past the wet bulb; insufficient ventilation can lead to overestimation of humidity (the wet bulb reads too high).
  13. Gatley, D. P. (2004). Psychrometric chart celebrates 100th anniversary (ASHRAE Journal 46(11)); bibliographic record and summary. ASHRAE Journal, November 2004; record in the International Institute of Refrigeration FRIDOC database. journal article (record) Reviews psychrometrics before 1904, when Willis H. Carrier produced the first psychrometric chart, and the chart's development to 2004.