露点温度与绝对湿度计算器
指导
露点温度与绝对湿度计算器
Enter the air temperature and relative humidity and the tool returns the dew point, wet-bulb temperature, NOAA heat index, saturation and actual vapor pressure, absolute humidity in g/m³, mixing ratio, specific humidity, moist air density, and a plain-language comfort rating. Numbers update as you type, no clicks or server round-trips required.
The formulas are the same ones used in HVAC sizing, weather forecasting, greenhouse and drying-room control, brewery and bakery process work, and outdoor-event safety planning. Toggle the units between Celsius and Fahrenheit at the top — the temperature you already entered is converted, not erased.
如何使用
- Pick your unit — Celsius or Fahrenheit. All inputs and outputs follow the same unit.
- Type the dry-bulb (ambient) air temperature.
- Type the relative humidity as a percentage between 0 and 100.
- Optional: adjust the air pressure in hectopascals (hPa). The default 1013.25 hPa is sea-level standard pressure; lower it for high-altitude locations to make the mixing ratio, specific humidity, and air-density results match your environment.
- Read the results in the table on the right — all ten values recalculate on every keystroke.
特征
- Magnus-Tetens dew point – Uses the Alduchov & Eskridge 1996 coefficients (A=6.1094, B=17.625, C=243.04) — accurate to within 0.4% across the meteorological range of −40 °C to +50 °C.
- Stull wet-bulb temperature – Implements the 2011 Stull empirical formula, valid for sea-level pressure, −20 °C to 50 °C, and 5–99% relative humidity.
- NOAA heat index – Full Rothfusz regression with the low-humidity and high-humidity adjustment terms. The simple Steadman approximation is used when the apparent temperature falls below 80 °F, with a clear “n/a” label so you know when the heat index does not apply.
- Saturation and actual vapor pressure – Reported in hPa so you can plug them directly into psychrometric charts and HVAC enthalpy calculations.
- Absolute humidity in g/m³ – Derived from the ideal gas law using the specific gas constant for water vapor (Rv = 461.5 J/kg·K).
- Mixing ratio and specific humidity – Both returned in g/kg, with pressure factored in so the values are correct at altitude as well as sea level.
- Moist air density – Calculated as the sum of partial densities for dry air (Rd = 287.058 J/kg·K) and water vapor — useful for aviation density-altitude work and ventilation airflow sizing.
- Comfort classification – Maps the dew point to a plain-English rating from “dry / crisp” to “miserable / dangerous”, colored so you can spot oppressive readings at a glance.
- Unit-preserving toggle – Switching between °C and °F converts the value already in the temperature field instead of clearing it.
- 实时重新计算 – All math runs in your browser; nothing is sent to a server.
常问问题
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What does the dew point actually represent?
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.
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How is dew point different from relative humidity?
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.
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What is the wet-bulb temperature and why does it matter?
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.
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Why is the Magnus-Tetens formula preferred over older approximations?
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.
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When is the NOAA heat index not valid?
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.
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How is absolute humidity calculated from vapor pressure?
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.
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What is the difference between mixing ratio and specific humidity?
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.
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At what dew point does the air start to feel oppressive?
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.
