Wet bulb calculator
95 °F at 25% relative humidity, the dry heat where evaporative cooling works. Add your elevation to the wet bulb calculator: unlike the dew point, the wet bulb depends on air pressure.
Notice: For general informational purposes only. Heat index values assume shade and light wind and do not account for exertion or individual health. Follow National Weather Service heat advisories, and seek medical attention for symptoms of heat-related illness.
Wet-bulb temperature
- Dew point td = 243.12 x ln(e' / 6.112 f) / (17.62 - ln(e' / 6.112 f)), with e' = 14.1 hPa the vapor pressure of the air and f = 1.0047 the enhancement factor (WMO-No. 8, eq. 4.B.6). Cool this air by 41.3 °F and dew forms.
- 53.7 °F (12.0 °C)
- Relative humidity As entered. Relative to saturation over liquid water, which is how WMO says relative humidity is reported even below freezing.
- 25%
- Dew point reading In the words of the National Weather Service office at La Crosse, whose "general comfort levels using dew point" for the summer months are: 55 °F or less "dry and comfortable"; between 55 and 65 °F "becoming "sticky" with muggy evenings"; 65 °F or more "lots of moisture in the air, becoming oppressive". It is a summer scale.
- dry and comfortable
- Wet-bulb temperature The reading of a thermometer wrapped in a wet wick with air drawn past it: evaporation cools it until the heat taken from the air balances the heat carried off by the water. Solved from WMO's psychrometer equation e' = f ew(tw) - 0.000653 x (1 + 0.000944 tw) x p x (t - tw) at 1,013 hPa (eq. 4.B.8). It always sits between the dew point and the air temperature.
- 68.7 °F (20.4 °C)
- Heat index NWS's Rothfusz regression, used because the simple formula came to 80 °F or more. NWS: "The Rothfusz regression is not valid for extreme temperature and relative humidity conditions beyond the range of data considered by Steadman."
- 92.7 °F (33.7 °C)
- NWS heat index class NWS Amarillo's classification, effect on the body: "Heat stroke, heat cramps, or heat exhaustion possible with prolonged exposure and/or physical activity." The same page gives its values for shady locations: "If you are exposed to direct sunlight, the heat index value can be increased by up to 15°F."
- Extreme Caution
- Vapor pressure The partial pressure of the water vapor in the air. Saturation at 95.0 °F would be 56.39 hPa: ew(t) = 6.112 x exp(17.62 t / (243.12 + t)) hPa (eq. 4.B.1) times the moist-air enhancement factor f = 1.0047.
- 14.1 hPa (0.4163 inHg)
- Absolute humidity Grams of water vapor in each cubic meter of air: e' / (Rv x T), with Rv = R / (0.62198 x M_air) = 461.5 J/(kg K). It changes with temperature even when the dew point does not, because warm air takes up more room.
- 9.91 g/m3
- Humidity ratio Mass of water vapor per mass of dry air, r = 0.62198 x e' / (p - e'), at 1,013 hPa. Grains per pound is the same figure times 7, since a pound is 7,000 grains. This is the number on the vertical axis of a psychrometric chart, and the one HVAC load calculations use.
- 8.78 g/kg (61.4 gr/lb)
- Air pressure used The 1976 U.S. Standard Atmosphere at sea level. A real day's pressure moves a few percent with the weather, which shifts the wet bulb and the humidity ratio slightly and the dew point not at all.
- 1,013.3 hPa (29.92 inHg)
| Relative humidity | Dew point | Wet bulb | Heat index | Humidity ratio |
|---|---|---|---|---|
| 10% | 29.9 °F | 61.0 °F | 89.4 °F | 3.48 g/kg |
| 20% | 47.6 °F | 66.2 °F | 91.5 °F | 7.00 g/kg |
| 30% | 58.7 °F | 71.0 °F | 94.4 °F | 10.6 g/kg |
| 40% | 66.9 °F | 75.3 °F | 99.0 °F | 14.2 g/kg |
| 50% | 73.4 °F | 79.2 °F | 105.2 °F | 17.8 g/kg |
| 60% | 78.9 °F | 82.8 °F | 113.1 °F | 21.5 g/kg |
| 70% | 83.7 °F | 86.2 °F | 122.6 °F | 25.2 g/kg |
| 80% | 87.9 °F | 89.3 °F | 133.8 °F | 29.0 g/kg |
| 90% | 91.6 °F | 92.3 °F | 146.6 °F | 32.8 g/kg |
| 100% | 95.0 °F | 95.0 °F | 161.1 °F | 36.7 g/kg |
- Saturation, dew point, frost point, relative humidity and the psychrometer equation are WMO's Annex 4.B formulas (WMO-No. 8, Volume I, Chapter 4), valid for -45 to 60 °C over water and -65 to 0 °C over ice. WMO calls them sufficiently accurate for all normal meteorological applications.
- The heat index is the National Weather Service's, computed in °F as its Weather Prediction Center describes. It is for shade; it is not a measure of what any particular person will feel.
- Relative humidity here is with respect to liquid water at every temperature, as WMO prescribes for reporting.
- Not modeled: supersaturation, salt or other solutes, and the heat a real wet-bulb thermometer gains from sunlight or loses through poor ventilation.
How the Dew Point Calculator works
The wet-bulb temperature is what a thermometer reads with its bulb in a wet wick and air drawn past it. Water evaporates from the wick and cools it until the heat the air supplies balances the heat the evaporating water carries away. The drier the air, the faster the evaporation and the lower the reading. WMO-No. 8 gives the balance for the aspirated (Assmann) psychrometer, the reference instrument:
e = f x ew(tw) - 0.000653 x (1 + 0.000944 x tw) x p x (t - tw)
where e is the vapor pressure of the air, ew(tw) the saturation vapor pressure at the wet-bulb temperature, f the moist-air enhancement factor and p the air pressure, all in hPa and °C. There is no closed form for tw, so the page solves it numerically: the right-hand side rises steadily with tw, the answer lies between the dew point and the air temperature, and bisection finds it to well under a thousandth of a degree.
The worked example: 95 °F (35 °C) at 25% relative humidity at sea level. The air's vapor pressure is 14.10 hPa and its dew point 53.7 °F. The equation balances at tw = 20.36 °C: the saturated wick holds 1.0047 x 23.86 = 23.97 hPa, the psychrometer term is 0.000653 x 1.0192 x 1,013.25 x 14.64 = 9.87 hPa, and the difference is the 14.10 hPa the air already has. The wet bulb is 68.7 °F, and the wet-bulb depression (air temperature minus wet bulb) is 26.3 °F.
That depression is the number that matters for evaporative cooling. An evaporative (swamp) cooler cools air by evaporating water into it, along the same line a wet bulb follows, and no evaporative process can take air below its wet-bulb temperature. A real cooler gets part of the way there, so the wet bulb is the floor on what it can deliver, and the depression is the most it can take off. Cooling towers are rated the same way, by how close their outlet water gets to the wet bulb.
Pressure is in the equation, which is why the page asks for elevation. The same 95 °F and 25% has a wet bulb of 68.7 °F at sea level and 67.1 °F at 5,280 ft: thinner air carries less heat to the wick for the same evaporation, so the wick runs cooler. Calculators built on Stull's 2011 regression leave pressure out entirely; that formula was fitted at sea-level pressure and stated for relative humidities of 5% to 99% and temperatures of -20 to 50 °C.
Below freezing the wick usually freezes and the instrument reads the ice-bulb temperature, which WMO gives with 0.000575 in place of the water coefficient; the page reports both when the wet bulb is below 0 °C.
What it is not. The wet-bulb temperature is not the wet-bulb globe temperature (WBGT) used in occupational and athletic heat guidance, which also folds in sunshine through a black globe thermometer. And the reading of a psychrometer and the thermodynamic wet bulb on an engineer's psychrometric chart (the "temperature of adiabatic saturation") are close but not identical; WMO notes that the relation between them is a matter for controlled experiment.
Dew Point Calculator FAQ
How do you calculate wet-bulb temperature?
Solve WMO's psychrometer equation, e = f ew(tw) - 0.000653 (1 + 0.000944 tw) p (t - tw), for tw, where e is the air's vapor pressure from the temperature and relative humidity and p the pressure in hPa. It has no closed form, so it is solved numerically. At 95 °F and 25% relative humidity at sea level the answer is 68.7 °F.
Is wet-bulb temperature the same as dew point?
No. The wet bulb always lies between the dew point and the air temperature, and all three are equal only when the air is saturated. At 95 °F and 25% the dew point is 53.7 °F and the wet bulb 68.7 °F. The dew point measures the water in the air; the wet bulb measures how far evaporation can cool something in it.
How cold can an evaporative cooler make the air?
No colder than the wet-bulb temperature, which is the limit of cooling by evaporation. At 95 °F and 25% relative humidity that floor is 68.7 °F, a depression of 26.3 °F; a real cooler delivers some fraction of that depression, set by its pads and airflow.
Does altitude affect wet-bulb temperature?
Yes, slightly, because the air pressure is in the psychrometer equation. At 95 °F and 25% relative humidity the wet bulb is 68.7 °F at sea level and 67.1 °F at 5,280 ft. The dew point and relative humidity are not affected.
More from the Dew Point Calculator
Sources
- WMO Guide to Instruments and Methods of Observation (WMO-No. 8), Volume I, Chapter 4 'Measurement of humidity', preliminary 2018 edition: Annex 4.B 'Formulae for the computation of measures of humidity' eqs. (4.B.1)-(4.B.11) and its range statement; Annex 4.A eqs. (4.A.5), (4.A.6) and (4.A.13) for the 0.621 98 mass ratio; section 4.1.2 for the units (hPa, °C) (2018)
- NOAA/NWS Weather Prediction Center, 'The Heat Index Equation' (Rothfusz regression from NWS Technical Attachment SR 90-23, 1990, its low- and high-humidity adjustments, and the simpler Steadman-consistent formula for heat indices below about 80 °F); the 40 °F pass-through is from the WPC heat index calculator's own script (https://www.wpc.ncep.noaa.gov/html/heatindex.shtml) (1990)
- NWS Amarillo, TX, 'Heat Index' page: the 'Classification / Heat Index / Effect on the body' table, and the statements that chart values are for shady locations and that direct sunlight can raise the heat index by up to 15 °F (2026)
- NWS La Crosse, WI, 'Dew Point vs Humidity': 'General comfort levels USING DEW POINT that can be expected during the summer months' (2026)
- U.S. Standard Atmosphere, 1976 (NOAA, NASA, USAF; NOAA-S/T 76-1562, NASA-TM-X-74335), Part 1: Table 2 'Adopted constants' (g0, R*, P0, r0, T0, gamma), Table 3 (O2 fractional volume), Table 4 'The defined reference levels and gradients of the linearly segmented temperature-height profile' (H_b, L_M,b), section 1.2.4 (M0 = 28.9644 kg/kmol), section 1.3.1 eqs. (33a)/(33b), eq. (18) for geopotential altitude, eq. (42) for density, eq. (50) for speed of sound, Table 10 (sea-level values); Part 4 Table I 'Geopotential Altitude, Metric Units' for the printed base temperatures and pressures (1976)