Estimate dew point from air temperature and relative humidity using a liquid-water vapor-pressure approximation.
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%
0.01 – 100
Dew point
48.68615477 °F
Air temperature68 °F
Relative humidity50%
How this was calculated
γ = ln(RH/100) + 17.67T/(243.5+T); Td = 243.5γ/(17.67−γ), T in °C.
This estimates dew point relative to liquid water, not frost point or wet-bulb temperature.
Method
Set γ = ln(RH/100) + 17.67T/(243.5+T), with T in Celsius. Dew point Td = 243.5γ/(17.67−γ). This follows by substituting vapor pressure e = RH × es/100 into the inverted saturation-vapor-pressure relation.
Worked example
At 20°C and 50% relative humidity the estimated dew point is about 9.27°C. At 100% relative humidity, dew point equals air temperature.
Assumptions and limits
Inputs are limited to −40–60°C and humidity above 0%, up to 100%. Zero relative humidity has no finite dew point in this model. Below freezing, this is a liquid-water dew point, not frost point over ice.