Relative Humidity From Dew Point Calculator

Relative humidity is the ratio of the actual water vapor pressure to the saturation vapor pressure at the same temperature. Since dew point is by definition the temperature at which the present vapor pressure would be the saturation value, the same saturation function evaluated at Td gives e and evaluated at T gives es, so RH is simply 100 e / es. This tool uses the Magnus form es = 6.112 exp(17.67 T / (T + 243.5)) hPa, with coefficients popularized by Bolton in 1980, which reproduce reference tables to within about 0.1 percent between -30 and 50 degrees C. Nothing in that relation requires pressure, which is why RH computed from T and Td is essentially pressure independent; the entered pressure affects only the mixing ratio and absolute humidity reported alongside. Useful anchors: a 1 degree C dew point depression corresponds to roughly 93 to 95 percent RH at ordinary temperatures, a 10 degree C depression near 25 degrees C gives about 48 percent, and zero depression is saturation. Indoor guidance generally targets 40 to 60 percent RH, since below 30 percent respiratory irritation and static discharge increase while above 70 percent mould growth accelerates. Two caveats matter. Below freezing, saturation over ice is lower than over supercooled water, so RH with respect to ice exceeds RH with respect to water for the same air and any reported value must state its reference. Sensor accuracy also degrades at very low humidity and very low temperature, where errors of 2 to 3 percentage points are routine.

Instrumental hygrometry predates the theory. Horace Benedict de Saussure built a hair hygrometer in 1783 that exploited the way human hair lengthens as it absorbs moisture, and John Frederic Daniell introduced the dew point hygrometer in 1820, cooling a surface until dew appeared on it. Henri Victor Regnault improved the technique in the 1840s with precise condensation hygrometers and produced vapor pressure tables of lasting accuracy. Heinrich Gustav Magnus published his empirical exponential fit for saturation vapor pressure in 1844, and the general form still bears his name, sometimes credited jointly as August-Roche-Magnus. Relative humidity as a defined percentage was formalized as national weather services standardized observing practice in the late nineteenth and early twentieth centuries, with the sling psychrometer inside a Stevenson screen becoming the field standard. The modern coefficient sets came from twentieth century refits to laboratory data: Otto Tetens proposed a widely used form in 1930, David Bolton published the 6.112, 17.67 and 243.5 coefficients in Monthly Weather Review in 1980, and Oleg Alduchov and Robert Eskridge presented an improved Magnus approximation in the Journal of Applied Meteorology in 1996. The World Meteorological Organization guide to instruments and methods of observation codifies which formulation stations should apply so that reports remain comparable across networks.