Relative Humidity
Enter air temperature and dew-point, compute relative humidity RH = es(dew)/es(temp) × 100% via the Tetens saturation vapor-pressure formula, plus actual and saturation vapor pressure.
Input Data
Results
At a glance:Relative humidity (RH, %) is the most familiar humidity indicator, describing how 'wet' the air is. It is defined as the percentage of actual vapor pressure ea over the saturation vapor pressure es at that temperature: RH = ea/es × 100%. At RH = 100% the air is saturated (cooling further causes dew, fog or precipitation); lower RH means drier air (farther from saturation, larger evaporation potential). The most reliable way to compute RH is via dew-point temperature. Establish the concepts: air water vapor has a partial pressure, the 'actual vapor pressure ea'; the upper limit of vapor pressure at a temperature is the 'saturation vapor pressure es', varying only with temperature (higher T gives larger es). The 'dew-point temperature Td' is the temperature reached when air (with unchanged water amount) is cooled just to saturation, beginning condensation — so at the dew point, actual vapor pressure exactly equals the saturation vapor pressure at the dew point, ea = es(Td). The saturation vapor pressure at current air temperature T is es(T). Dividing the two gives RH: RH = ea/es(T) × 100% = es(Td)/es(T) × 100%. Both saturation vapor pressures use the Tetens formula common in agronomy and hydrology: es = 0.6108 × exp(17.27 × T ÷ (T + 237.3)) (kPa). Step by step: compute es(T) from T, es(Td) = ea from Td, divide and × 100 for RH. Example: T = 25°C, Td = 15°C: es(25) = 0.6108 × e^(17.27×25/262.3) ≈ 3.168 kPa; ea = es(15) = 0.6108 × e^(17.27×15/252.3) ≈ 1.705 kPa; RH = 1.705 ÷ 3.168 × 100 ≈ 53.8%. So this temperature/dew-point pair gives about 54% RH. If the dew point rises near air temperature (e.g. 24°C), RH approaches 100% (saturated, muggy); if far below, RH is low (dry). Uses: judge air dryness and comfort — at the same temperature, higher RH is more muggy (sweat evaporates poorly), lower is drier; assess crop and environmental water stress and disease risk — prolonged high RH favors fungal diseases (powdery mildew, botrytis), too low accelerates transpiration water loss; judge dew and fog formation (RH reaches 100% when air cools to dew point); and with temperature, compute vapor-pressure deficit VPD = es − ea for greenhouse control and evapotranspiration. Notes: dew point cannot exceed air temperature (RH = 100% when equal; this tool clamps RH to 0–100%); RH changes with temperature — same water amount, higher T raises es and lowers RH (why RH falls by day, rises by night); if you have wet/dry-bulb or vapor pressure, the conversion differs slightly — this tool uses air temp + dew point; Tetens is for water surface, ice surface below 0°C needs separate handling; measure dew point with a reliable dew-point hygrometer or derive from wet/dry bulb.
Formula
Relative humidity: RH = ea ÷ es × 100%.
Actual vapor pressure ea = es(dew point Td); saturation es = es(air temp T).
Saturation vapor pressure by Tetens: es = 0.6108·exp(17.27·T/(T+237.3)).
$$RH = \dfrac{e_s(T_d)}{e_s(T)} \times 100\%$$How to Use
- Enter current air temperature T (°C).
- Enter dew-point temperature Td (°C, not above air temperature).
- The tool returns RH and lists actual and saturation vapor pressure.
RH at different dew points for air temperature 25°C
| Dew point Td (°C) | Actual vapor pressure ea (kPa) | Relative humidity RH (%) |
|---|---|---|
| 25 | 3.168 | 100 |
| 20 | 2.338 | 73.8 |
| 15 | 1.705 | 53.8 |
| 10 | 1.228 | 38.8 |
| 5 | 0.872 | 27.5 |
Closer dew point to air temperature gives higher RH (more humid); Td = T means saturation, RH = 100%.
Case Studies
RH on a muggy Hong Kong afternoon
Summer afternoon: T = 25°C, Td = 15°C.
es(25) ≈ 3.168 kPa, ea = es(15) ≈ 1.705 kPa.
RH = 1.705/3.168 × 100 ≈ 53.8%, moderate and fairly dry; if Td rises to 22°C, RH reaches about 84% and feels muggy.
Night cooling raises RH
Same water amount (dew point fixed 15°C), night temperature drops from 25°C to 16°C.
es drops from 3.168 to about 1.818 kPa, while ea stays 1.705 kPa.
RH = 1.705/1.818 × 100 ≈ 93.8%, near saturation, so dew and fog are common at dawn.
FAQ
What is the relation between dew point and relative humidity?
Dew point is the temperature reached when air is cooled just to saturation, reflecting the actual water content (absolute humidity). Relative humidity is the percent of actual vapor pressure over saturation vapor pressure at current temperature. The relation is RH = es(dew)/es(temp) × 100%. Closer dew point to temperature gives higher RH (RH = 100% when equal); lower dew point gives lower RH. Dew point is a stable humidity measure (independent of temperature), while RH varies with temperature — which is why this tool computes RH from dew point and air temperature.
Why is RH low by day and high at night?
Within a day, if the water amount (dew point) is roughly constant, RH varies opposite to temperature. Daytime warming raises saturation vapor pressure es, so the same actual vapor pressure ea makes a smaller fraction — RH drops; nighttime cooling lowers es, RH rises, even reaching the dew point and causing dew/fog. That is why dew and fog appear at dawn and afternoons feel drier. This tool can take different temperatures at fixed dew point to see RH vary with temperature.
Can dew point be higher than air temperature?
No. Dew point is the temperature air reaches when cooled to saturation, at most equal to current air temperature (already saturated, RH = 100%), never above. If measured data show dew point above air temperature, it is usually measurement error or bad data. This tool clamps RH to 0–100%; entering dew point equal to or above air temperature shows RH = 100%. Normally dew point should be ≤ air temperature.
How does high RH affect crops?
Prolonged high RH (near saturation) suppresses plant transpiration and nutrient transport with the transpiration stream; more importantly it raises fungal and bacterial disease risk — high humidity with suitable temperature easily triggers powdery mildew, botrytis, downy mildew, and leaf dew further aids spore germination. So greenhouses and dense planting need ventilation and dehumidification and night-dew control. Conversely too low RH accelerates transpiration and water stress. Management often uses VPD (vapor-pressure deficit) as a plant-relevant indicator.
Can I compute RH from wet/dry-bulb temperatures?
Yes, but the method differs slightly. Dry-bulb is air temperature, wet-bulb reflects evaporative cooling; from the wet/dry-bulb difference with a psychrometric table (or constant) you get actual vapor pressure, divided by dry-bulb saturation es gives RH. This is conceptually the same as the tool's 'RH from dew point' — only the route to get ea differs. If you have wet/dry-bulb data, convert to dew point or vapor pressure first, then use this tool or a psychrometric table.
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References
Content review: Calculatorism Science Team. Results are for reference only; please refer to the relevant authorities for the official figures.