Psychrometric Dew Point Calculator

Dew point is the temperature to which moist air must be cooled, at constant pressure and constant moisture content, for its water vapor to reach saturation. It is therefore a direct proxy for absolute moisture, and unlike relative humidity it does not move when air is merely heated or cooled. The calculation runs through saturation vapor pressure. The Magnus form used here, Ps equals 0.61078 times the exponential of 17.27T divided by T plus 237.3, gives kilopascals for T in degrees C and is accurate to a few tenths of a percent between roughly minus 20 and 50 degrees C. Multiplying by relative humidity gives actual vapor pressure, and inverting the same expression returns the temperature at which that pressure would be saturation. Wet bulb, reported alongside, is the lowest temperature reachable by evaporative cooling and always lies between dew point and dry bulb; the closed-form fit used here assumes pressures near sea level. Practical thresholds: dew points below 55 degrees F feel dry, 60 to 65 degrees F feels noticeably humid, and above 70 degrees F is oppressive. Indoor design of 75 degrees F at 50 percent relative humidity corresponds to a dew point near 55 degrees F and about 65 grains of moisture per pound of dry air. Altitude matters for humidity ratio, enthalpy, and specific volume, since Denver at 5,280 feet sits about 17 percent below sea-level pressure, but dew point itself depends only on vapor pressure. The key limitation is that these are equilibrium properties of well-mixed air, so any surface colder than the calculated dew point will sweat.

Dew point began as a measurement rather than a formula. John Frederic Daniell described a hygrometer in 1820 in which evaporating ether chilled a polished bulb until dew appeared, giving a direct reading of the condensation temperature, and Henri Victor Regnault refined the chilled-mirror principle in the 1840s into the reference method that modern optical dew-point hygrometers still use. In parallel the saturation vapor pressure curve was being pinned down experimentally: Heinrich Gustav Magnus published his exponential fit in 1844, and later workers, among them Otto Tetens in 1930, adjusted the coefficients, producing the family of August-Roche-Magnus expressions applied in this calculator. Turning those relationships into a design tool was largely the work of Willis Carrier, whose 1911 paper Rational Psychrometric Formulae, delivered to the American Society of Mechanical Engineers, tied dew point, vapor pressure, enthalpy, and wet-bulb temperature into one consistent system and introduced the chart form engineers still read. Richard Mollier developed the enthalpy-based diagram that became the parallel convention in continental Europe. ASHRAE later took custody of the standard formulations in its Handbook of Fundamentals, while national metrology institutes established the humidity scales against which instruments are calibrated. Explicit approximations for wet bulb from dry bulb and relative humidity are far more recent; the widely cited closed-form fit was published by Roland Stull in 2011 for meteorological work where iterating the exact relation was inconvenient.