Dew Point Depression Calculator

Dew point depression is the arithmetic difference between air temperature and dew point, and it measures how much cooling the air must undergo before it saturates. Its value as a diagnostic comes from what the two temperatures represent: air temperature sets the saturation vapor pressure, while dew point fixes the actual vapor pressure and hence the absolute moisture content. Their difference therefore encodes both quantities at once. The Magnus form used here, with coefficients 17.625 and 243.04 degrees C, converts between the two and returns relative humidity as the ratio of the exponentials, accurate to a few tenths of a percent across normal surface conditions. Subtracting actual from saturation vapor pressure yields the vapor pressure deficit, the drying power that drives evaporation. Practitioners read the depression against fairly stable thresholds. Below about 1 degree C, radiation fog and dew are imminent; 1 to 3 degrees at sunset with clear skies and light wind makes overnight fog probable; values above 10 degrees indicate a dry air mass. Fire weather agencies issue Red Flag conditions when relative humidity falls below roughly 15 percent alongside sustained winds near 25 mph, which corresponds to very large depressions. Horticultural practice generally targets a vapor pressure deficit near 8 to 12 hPa for greenhouse crops. The main caveats are that depression is temperature dependent, so the same 5 degree spread means different relative humidity at 10 and 30 degrees C, and that the wet bulb value returned here uses the Stull 2011 approximation, which assumes near sea level pressure.

Dew point as a measurable quantity dates to John Frederic Daniell, who described his dew point hygrometer to the Royal Society in 1820. The instrument used ether evaporating inside a thin glass bulb to chill an adjacent polished surface until dew appeared, with the temperature at that instant read directly as the dew point. It was the first device to give the quantity a defensible physical definition rather than an indirect proxy, unlike the hair hygrometer Horace-Benedict de Saussure had introduced in 1783. Ernst Ferdinand August developed the psychrometer in the 1820s, and Richard Assmann standardized the aspirated version in 1887, giving observers a rugged field method that infers dew point from paired dry and wet bulb readings. The conversion mathematics came from Heinrich Gustav Magnus, whose 1844 empirical fit for saturation vapor pressure over water is the ancestor of the coefficients used in this calculator and in most weather software today. Arden Buck refined the constants in 1981, and the World Meteorological Organization codified observing practice for humidity in its guide to instruments and methods of observation. Two later developments extended the metric beyond weather reporting: plant scientists adopted vapor pressure deficit as the controlling variable for stomatal behavior and irrigation scheduling, and Roland Stull published a practical closed-form wet bulb approximation in 2011 that made heat stress calculations feasible without iteration.