Dew point: the condensation temperature

The dew point (Td) is the temperature to which air must cool, at constant pressure and without adding or removing moisture, for water vapor to start condensing. When Td equals air temperature T, relative humidity is 100% and condensation forms (fog, dew, frost). The Magnus-Tetens formula, in the refined Alduchov-Eskridge (1996) version, is the most accurate for meteorological use: γ = ln(RH/100) + (a·T)/(b+T) and Td = b·γ/(a − γ), with a=17.625 and b=243.04, valid for −45°C ≤ T ≤ 60°C with error <0.4%.

Saturation vapor pressure and humidity

Saturation vapor pressure e_s (hPa) follows Clausius-Clapeyron, approximated by Magnus: e_s(T) = 6.112 · exp[17.625·T/(T+243.04)]. Relative humidity is RH = e/e_s · 100%, where e is actual partial vapor pressure. Absolute humidity AH (g/m³) at T and RH: AH = e·M_w/(R·T_K) ≈ (e·216.7)/(T+273.15), with M_w=18.02 g/mol and R=8.314 J/(mol·K).

Why dew point is a comfort indicator

Relative humidity depends on temperature, while Td does not: it is an absolute measure of vapor content. Td below 10°C: very dry air, excellent comfort. 10-15°C: pleasant. 15-18°C: slight noticeable humidity. 18-21°C: growing muggy. 21-24°C: very uncomfortable, ineffective sweating. Above 24°C: oppressive, heat stroke risk. NWS uses Td ≥ 24°C as extreme mugginess alert threshold.

Heat index: perceived temperature

The heat index combines T and RH into an apparent temperature, Rothfusz (NWS 1990): HI = −8.785 + 1.611·T + 2.339·RH − 0.146·T·RH − 0.01231·T² − 0.01642·RH² + 0.002212·T²·RH + 0.000725·T·RH² − 0.00000358·T²·RH² (T in °C, RH in %). Valid for T ≥ 27°C and RH ≥ 40%. NWS categories: 27-32°C caution; 32-41°C extreme caution; 41-54°C danger; >54°C extreme danger (heat stroke in minutes).

Magnus formula variants

Alduchov-Eskridge (1996): a=17.625, b=243.04°C (WMO standard, 0.4% error). Original Magnus 1844: a=17.27, b=237.7°C. Arden Buck 1981: more accurate but complex (T>0 vs T<0). Sonntag 1990: a=17.62, b=243.12°C (DWD standard). Differences <0.1°C on Td under normal conditions. Above 50°C or below −40°C use Wexler-Greenspan (ITS-90).

Applications

Meteorology (fog, frost, visibility forecasts); HVAC (targeted dehumidification, ASHRAE 55); preservation (optimal RH libraries 30-50%, museums 45-55%, wine 70-80%); food industry (water activity, microbiological stability); building science (condensation risk on cold walls, EN 13788); aviation (carburetor icing, surface icing); greenhouses (ventilation to avoid mold at RH>85%); offset printing (papers, inks at RH=50%); electronics (condensation on cold devices exposed to warm humid air).

How to use

Select mode: Magnus (T, RH → Td), RH from Td (T, Td → RH), Absolute humidity (T, RH → AH, vapor pressure), Comfort (T, RH → Td, heat index, comfort class). All SI units, T in °C, RH in %.

The Magnus formula and modern meteorology

The dew-point calculation is based on the saturation-vapour-pressure equation formulated by Heinrich Gustav Magnus in 1844 and refined through the twentieth century. The coefficients a = 17.625 and b = 243.04 used by the calculator come from the Alduchov-Eskridge revision of 1996, now standard in weather services. In aviation the spread between temperature and dew point flags the risk of fog and icing, while in climatology it is a routine derived quantity.

Practical applications of the dew point

Knowing the dew point is essential in construction to prevent interstitial condensation and mould on thermal bridges, in the conservation of artworks and archives, in HVAC design and in the food and pharmaceutical industries, where controlled humidity is a process parameter. In agriculture the closeness of the night-time temperature to the dew point signals dew and frost formation, crucial for protecting crops.

Updated July 2026