Dew Point and Humidity calculations

Dew Pt. from Humidity

Dew Point : °C

Dew Pt. from WetBulb

Dew Point : °C

Humidity : %


These calculations are based on Vaisala's humidity conversion formulas.

Note that measuring humidity accurately is challenging, especially at high humidities. Even the best humidity sensors start to over read 1 - 3 % once you get past 90% RH.

  1. Initially the Water vapour saturation pressure (Pws) is calculated:
  2. $$P_{ws}=A*10^{\left ( \frac{mT}{T+T_n} \right ) }$$

    T is the temperature in deg C, the following are constants valid for a temperature range -20 - +50 deg C with a maximum error of 0.083%; A = 6.116441, m = 7.591386, Tn = 240.7263.


  3. Water vapour pressure itself is then calculated (hPa) using the relative humidity and Pws

  4. $$P_w = P_{ws} * \frac{RH}{100}$$

  5. Now Dew Point (Td) can be calculated:
  6. $$T_d = \frac{T_n}{\left [ \frac{m}{^{10}log \left (\frac{P_w}{A} \right )} -1 \right ]}$$


  7. For calculating dew point and humidity from the wet bulb (for a given pressure):

  8. $$P_w = P_{ws}(T_{wet})-pressure * K * (T_{dry}-T_{wet})$$


    where Pw = Water vapour pressure, K = Psychrometer constant 0.0008 /deg C. Once Pw is known, humidity and dew point can be calculated using the above equations.



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