Calculateur de point de rosée et d'humidité absolue

The dew point is the temperature to which a parcel of air must be cooled at constant pressure and constant water-vapor content for saturation to occur. Below that temperature, water vapor condenses into liquid — dew on grass, fog in valleys, droplets on a cold glass. Unlike relative humidity, the dew point is an absolute measure of how much moisture is in the air, which is why meteorologists use it to compare two days that feel different despite having the same RH reading.

Relative humidity is a ratio: actual vapor pressure divided by the saturation vapor pressure at the current temperature, expressed as a percent. Because saturation vapor pressure rises sharply with temperature, the same air mass can swing from 80% RH at dawn to 40% RH by noon without a single water molecule entering or leaving. Dew point only changes when moisture is added or removed, so it is a more stable indicator of how the air actually feels.

The wet-bulb temperature is what a thermometer would read if its bulb were wrapped in a wet cloth and ventilated. Evaporation cools the bulb until it reaches equilibrium with the surrounding air. The wet-bulb temperature is the lowest temperature achievable by evaporative cooling, which is why it is the operating limit for swamp coolers, the survival threshold for human heat tolerance (~35 °C), and a critical input to cooling-tower design.

The Magnus form expresses saturation vapor pressure as e_s = A·exp(B·T/(C+T)). Older coefficients (Tetens 1930, August 1828) were tuned to narrow temperature ranges. Alduchov & Eskridge published refined coefficients in 1996 (A=6.1094, B=17.625, C=243.04) that hold to within 0.4% from −40 °C to +50 °C — well past meteorological needs. The formula is closed-form, monotonic, and cheap to evaluate, which is why almost every modern weather station and HVAC controller uses some variant of it.

The Rothfusz regression that powers the National Weather Service heat index was fit to U.S. summer conditions. It assumes a healthy adult in the shade, light wind, and ambient air temperature above about 27 °C (80 °F). Below that threshold, evaporative cooling dominates and the regression returns numbers that are statistically meaningless, so the tool falls back to a simple Steadman approximation and flags the result as not applicable.

Absolute humidity is the mass of water vapor per unit volume of air. Starting from the ideal gas law for water vapor (p_v · V = n · R · T), substituting molar mass M_v = 18.016 g/mol and using the specific gas constant Rv = 461.5 J/kg·K, the working formula becomes AH = e / (Rv · T), where e is the actual vapor pressure in pascals and T is the temperature in kelvin. The result comes out in kg/m³; multiply by 1000 for g/m³.

Mixing ratio (w) is the mass of water vapor per mass of dry air; specific humidity (q) is the mass of water vapor per mass of moist air (vapor + dry air). They are related by q = w / (1 + w) and differ by under 2% in normal atmospheric conditions, but the distinction matters for high-precision applications: mixing ratio is conserved during adiabatic motion that does not involve condensation, which makes it the preferred coordinate for thermodynamic diagrams like the Skew-T log-P.

Comfort surveys consistently show a sharp break around a dew point of 16 °C (60 °F). Below 13 °C most people describe the air as pleasant; from 13–16 °C it is noticeably moist but tolerable; above 18 °C sweat stops evaporating efficiently and exertion becomes harder; above 21 °C the air feels heavy even at rest; above 24 °C heat injury risk climbs rapidly during physical work. The tool uses these breakpoints to assign the color-coded comfort rating.