Dew Point and Humidity Calculator
Find the dew point, relative humidity or temperature from the other two. Made for pilots, and just as useful for weather, comfort, damp walls and painting.
Both agree within about 0.1 °C in everyday weather. Buck is a little closer to the reference tables in the heat.
The actual pressure where you are (QFE), not the sea-level QNH. It only changes the mixing ratio, specific humidity, enthalpy and pressure wet-bulb.
A window, wall, pipe, steel to be painted or a cold drink. Leave blank to skip.
How your dew point is worked out
Warm air can hold more water vapour. Compare what it holds with what it could hold.
- 1
Saturation pressure
es at 25 °C
= 31.62 hPa - 2
Water in the air
60% × 31.6 hPa
= 18.97 hPa - 3
Find the dew point
es(dew pt) = 19.0
→ 16.7 °C - 4
What it means
Sticky
Cloud ~3,400 ft
Read more: the formula, a worked example, comfort and sources
Warm air can hold more water vapour than cold air. The most it can hold at a temperature shows up as the saturation vapour pressure. The water actually in the air gives the vapour pressure, and the dew point is simply the temperature whose saturation vapour pressure equals it. Relative humidity is the ratio of the two pressures. So any two of temperature, dew point and humidity give the third.
The formula
saturation vapour pressure (hPa) = 6.1094 × e^(17.625 × T ÷ (243.04 + T))
vapour pressure = relative humidity ÷ 100 × saturation vapour pressure at T
dew point = 243.04 × g ÷ (17.625 − g), where g = ln(vapour pressure ÷ 6.1094)
relative humidity = 100 × saturation vapour pressure at the dew point ÷ saturation vapour pressure at T
absolute humidity (g/m³) = 216.68 × vapour pressure ÷ (T + 273.15)
mixing ratio (g/kg) = 622 × vapour pressure ÷ (station pressure − vapour pressure)
cloud base (m) ≈ 125 × (T − dew point)
T is in °C. This is the Magnus formula with the Alduchov and Eskridge (1996) constants, the same as our cloud base calculator and carb icing calculator. In Advanced you can switch to Buck (1981): 6.1121 × e^((18.729 − T ÷ 227.3) × T ÷ (257.87 + T)), which we solve numerically for the dew point.
Worked example
Air at 25 °C with 60% relative humidity, at sea-level pressure (1013.25 hPa), and a wall at 20 °C.
- Saturation vapour pressure: 17.625 × 25 ÷ 268.04 = 1.64388. e^1.64388 = 5.17520, × 6.1094 = 31.62 hPa.
- Vapour pressure: 0.60 × 31.6174 = 18.97 hPa.
- Dew point: g = ln(18.9704 ÷ 6.1094) = ln 3.10512 = 1.13305. 243.04 × 1.13305 = 275.377, and 17.625 − 1.13305 = 16.4919, so 275.377 ÷ 16.4919 = 16.7 °C (62.1 °F).
- Absolute humidity: 216.68 × 18.9704 ÷ 298.15 = 13.8 g/m³.
- Mixing ratio: 622 × 18.9704 ÷ (1013.25 − 18.9704) = 11,799.6 ÷ 994.28 = 11.9 g/kg.
- Spread: 25 − 16.70 = 8.30 °C, so the cumulus base is about 125 × 8.30 = 1,038 m, or about 3,400 ft above the ground.
- Comfort: a dew point of 62 °F is between 55 and 65 °F: becoming sticky.
- Wet-bulb (Stull): 19.5 °C.
- Wall at 20 °C (Advanced): 20 − 16.7 = 3.3 °C above the dew point, so no water forms, but the humidity right at the wall is 18.97 ÷ 23.33 (saturation at 20 °C) = 81%: above 80%, so there is a mould risk if it stays like that.
Comfort by dew point (general guide)
| Dew point | How it feels |
|---|---|
| 55 °F (12.8 °C) or lower | Dry and comfortable |
| 55–65 °F (12.8–18.3 °C) | Becoming sticky, muggy evenings |
| 65 °F (18.3 °C) or higher | Muggy and oppressive |
Wet-bulb temperature
The wet-bulb is the lowest temperature you can reach by evaporating water into the air, the reading of a thermometer wrapped in a wet cloth. The quick figure uses Stull’s (2011) formula, made for sea-level pressure, humidity of 5–99% and −20 to 50 °C; outside that range it can be more than 1 °C out, and we flag it. Advanced also solves the psychrometer equation at your station pressure: vapour pressure = saturation vapour pressure at the wet-bulb − 0.000653 × (1 + 0.000944 × wet-bulb) × pressure × (T − wet-bulb).
Notes and sources
- Alduchov, O. A. and Eskridge, R. E. (1996), Improved Magnus form approximation of saturation vapor pressure, J. Appl. Meteor. 35, 601–609. Frost point uses their ice constants: 6.1121 × e^(22.587 T ÷ (273.86 + T)).
- Buck, A. L. (1981), New equations for computing vapor pressure and enhancement factor, J. Appl. Meteor. 20, 1527–1532. We leave out the small enhancement factor (about 0.4%).
- Stull, R. (2011), Wet-bulb temperature from relative humidity and air temperature, J. Appl. Meteor. Climatol. 50, 2267–2269.
- Comfort bands: the US National Weather Service’s simple dew point guide. A general guide only; people used to hot, humid weather often feel fine higher up.
- Mould: ISO 13788 and BS 5250 design for surface humidity below 80%. Painting: ISO 8502-4 asks for the surface to be at least 3 °C above the dew point.
- Cloud base: about 125 m (400 ft) per °C of spread for cumulus from surface heating. It doesn’t work for fog, frontal or layer cloud. See the cloud base calculator for freezing level and more.
Common questions
What is the dew point?
The dew point is the temperature the air would have to cool to (at the same pressure and moisture) for it to become saturated, so that dew, fog or cloud forms. Unlike relative humidity it does not change when the air warms or cools during the day, so it is the best single measure of how much water vapour is in the air.
What is the difference between dew point and relative humidity?
Relative humidity is how full the air is compared with the most water vapour it could hold at its current temperature. Dew point is an amount: the higher it is, the more water vapour. On a cool morning the humidity can be 95% with a dew point of 10 °C, and in the afternoon 45% with the same dew point: the moisture hasn’t changed, only the temperature.
What dew point feels comfortable?
As a general guide from the US National Weather Service, a dew point of 55 °F (about 13 °C) or lower feels dry and comfortable, 55–65 °F (13–18 °C) becomes sticky with muggy evenings, and 65 °F (18 °C) or higher feels oppressive. People who live in hot, humid places often feel fine with higher dew points.
How do pilots use the dew point?
In a METAR the temperature and dew point appear together, like 25/17. The gap between them (the spread) tells you how close the air is to saturation. A spread of 2–3 °C or less warns of fog or low cloud, and the spread roughly gives the base of cumulus cloud: about 400 ft per °C, or 125 m per °C.
When does condensation form on a window or wall?
Water condenses on any surface that is at or below the dew point of the air next to it. Mould can grow even before that, when the humidity right at the surface stays at 80% or more for days. Warming the surface (insulation, heating) or drying the air (ventilation, a dehumidifier) lowers the risk. For painting and coatings the surface should be at least 3 °C above the dew point.
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