Compute dew point temperature from RH&T

Relative humidity (RH) and temperature (T) data from an RH&T sensor like the DHT22 can be used to compute not only absolute humidity AH but also dew point temperature T_D

There has been a fair amount of interest in my formula which computes absolute humidity (AH) from measured relative humidity (RH) and temperature (T) since it adds value to the output of RH&T sensors. To further extend this value, I have developed another formula which computes dew point temperature T_D from measured RH and T.

Formula for computing dew point temperature T_D
In this formula (P Mander 2017) the measured temperature T and the computed dew point temperature T_D are expressed in degrees Celsius, and the measured relative humidity RH is expressed in %

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Strategy for computing T_D from RH and T

1. The dew point temperature T_D is defined in the following relation where RH is expressed in %

2. To obtain values for P_sat, we can use the Bolton formula^{[REF, eq.10]} which generates saturation vapor pressure P_sat (hectopascals) as a function of temperature T (Celsius)

These formulas are stated to be accurate to within 0.1% over the temperature range –30°C to +35°C

3. Substituting in the first equation yields

Taking natural logarithms

Rearranging

Separating T_D terms on one side yields

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P Mander August 2017