Calculation
Es=6.112·e^(17.62·T/(243.12+T)); e=RH/100·Es; Td=243.12·ln(e/6.112)/(17.62−ln(e/6.112))
Dew point is the temperature at which air would just be saturated at unchanged moisture content; it is always at or below air temperature.
Magnus-Formel (Sonntag 1990, WMO-empfohlen, vom DWD verwendet)
Dew point temperature via the Magnus formula
Dew point is the temperature at which air would just be saturated at unchanged moisture content; it is always at or below air temperature.
MINTSI
02
Result
Select a target and calculate.
Understand the inputs
- Dew point temperature Td — Temperature to which air would need to cool, at unchanged moisture content, to reach saturation.
- Air temperature T — Currently measured air temperature.
- Relative humidity RH in % — Relative humidity at air temperature T, as a percentage between 0 and 100.
Example
T=20 °C and RH=60% give a dew point Td≈12.0 °C.
Assumptions and limits
Valid for flat water surfaces at temperatures at or above 0 °C (Sonntag coefficients); over ice below 0 °C different constants apply. No altitude correction of air pressure.
Understand Dew point temperature via the Magnus formula
This calculator determines dew point temperature from air temperature and relative humidity using the Magnus approximation used by the German Weather Service (DWD), or conversely relative humidity from air temperature and dew point.
What does this quantity describe?
Saturation vapour pressure follows from Es=6.112·e^(17.62·T/(243.12+T)) (T in °C, Es in hPa). Actual vapour pressure is e=RH/100·Es; solving backward gives dew point Td=243.12·ln(e/6.112)/(17.62−ln(e/6.112)).
Formula and variables
Es=6.112·e^(17.62·T/(243.12+T)); e=RH/100·Es; Td=243.12·ln(e/6.112)/(17.62−ln(e/6.112))
Es = 6.112·e^(17.62·T/(243.12+T))Td = 243.12·ln(e/6.112)/(17.62−ln(e/6.112))
| Symbol / input | Meaning |
|---|---|
| Dew point temperature Td | Temperature to which air would need to cool, at unchanged moisture content, to reach saturation. |
| Air temperature T | Currently measured air temperature. |
| Relative humidity RH in % | Relative humidity at air temperature T, as a percentage between 0 and 100. |
Choose the inputs correctly
T is the current air temperature, RH relative humidity in percent. Alternatively, RH can be calculated from T and a known dew point Td.
How to use the calculator
Take T and RH from a weather station, humidity sensor or measurement; for the reverse direction, use Td from a measurement at a cold surface.
Worked example
T=20 °C and RH=60% give a dew point Td≈12.0 °C.
Understand the result and units
The closer air temperature is to dew point, the higher relative humidity is; at T=Td the air is just saturated (RH=100%).
T and Td are temperatures, RH a percentage between 0 and 100.
Typical applications
Estimating condensation and mould risk on cold surfaces (windows, thermal bridges), dew-point monitoring in HVAC and compressed-air systems, and weather observation.
Assumptions, limits and common mistakes
Valid for flat water surfaces at temperatures at or above 0 °C (Sonntag coefficients, WMO-recommended); different constants apply over ice below 0 °C. No altitude or air-pressure correction.
Common mistake: Do not confuse the ice coefficients with the water coefficients when considering temperatures below 0 °C; this calculator uses only the water coefficients.
Frequently asked questions
What is “Dew point temperature via the Magnus formula” used for?
Estimating condensation and mould risk on cold surfaces (windows, thermal bridges), dew-point monitoring in HVAC and compressed-air systems, and weather observation.
Where do the input values come from?
T is the current air temperature, RH relative humidity in percent. Alternatively, RH can be calculated from T and a known dew point Td.
What does the result not cover?
Valid for flat water surfaces at temperatures at or above 0 °C (Sonntag coefficients, WMO-recommended); different constants apply over ice below 0 °C. No altitude or air-pressure correction.