How to use this calculator
- Enter or choose find.
- Enter or choose air temperature.
- Enter or choose temperature unit.
- The result updates as you type. Use Copy result or Share to keep it.
Method
γ = ln(RH/100) + aT ÷ (b + T); Td = bγ ÷ (a − γ); RH = 100 × exp(aTd ÷ (b + Td) − aT ÷ (b + T)), with a = 17.625 and b = 243.04 °C
- T
- air temperature in °C
- RH
- relative humidity in %
- Td
- dew point in °C
- a, b
- Magnus coefficients (Alduchov and Eskridge, 1996)
Worked example
- γ = ln(0.60) + 17.625 × 25 ÷ (243.04 + 25) = −0.5108 + 1.6438 = 1.1330.
- Td = 243.04 × 1.1330 ÷ (17.625 − 1.1330) = 16.7 °C.
Air at 25 °C and 60% relative humidity has a dew point of about 16.7 °C (62 °F).
What the result means
Surfaces colder than the dew point get wet with condensation, which is why cold windows and pipes drip. As a rough guide, many people find dew points above about 16 °C (60 °F) muggy and above about 21 °C (70 °F) oppressive.
Assumptions and limits
- Over liquid water, at normal atmospheric pressure.
- The Magnus formula is accurate to about 0.1 °C between −40 °C and 50 °C; outside that range use a more detailed equation.
- Below 0 °C the frost point (over ice) is slightly higher than the dew point.
Questions
Why is dew point a better measure of mugginess than humidity?
Relative humidity depends on temperature: 60% at 10 °C is dry-feeling air, while 60% at 30 °C is very humid. The dew point measures the moisture itself.
Can the dew point be higher than the temperature?
No. When the two are equal the air is saturated (100% relative humidity) and fog or dew forms.