Dew Point and Humidity
Dew point from humidity:
Dew point from wet bulb:
These calculations are based on Vaisala's humidity conversion formulas.
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.
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.
Initially the Water vapour saturation pressure (Pws) is calculated: $$P_{ws}=A*10^{\left ( \frac{mT}{T+T_n} \right ) }$$ T is the temperature in deg C, the following are constants valid for a temperature range -20 to +50 deg C with a maximum error of 0.083%; A = 6.116441, m = 7.591386, Tn = 240.7263 Water vapour pressure itself is then calculated (hPa) using the relative humidity and Pws : $$P_w = P_{ws} * \frac{RH}{100}$$ Now Dew Point (Td) can be calculated: $$T_d = \frac{T_n}{\left [ \frac{m}{^{10}log \left (\frac{P_w}{A} \right )} -1 \right ]}$$ For calculating dew point and humidity from the wet bulb (for a given pressure): $$P_w = P_{ws}(T_{wet})-pressure * K * (T_{dry}-T_{wet})$$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.