- Enter the measured air temperature and choose its unit.
- Enter relative humidity as a percentage between 0 and 100.
- Select a phase reference or keep Auto for typical weather use.
- Pick a constant set if you want to compare fits.
- Press Calculate to see results above.
This calculator uses a Magnus-type approximation. First compute: γ = ln(RH/100) + (a·T)/(b+T) where T is air temperature in °C and RH is relative humidity.
Dew point temperature is: Td = (b·γ)/(a−γ) using constants a and b from the selected fit.
Vapor pressure values use es(T)=6.112·exp((a·T)/(b+T)) in hPa, and e=(RH/100)·es(T).
| Air Temp (°C) | RH (%) | Dew Point (°C) | Depression (°C) | Actual Vapor Pressure (hPa) |
|---|---|---|---|---|
| 30 | 70 | 23.9 | 6.1 | 29.7 |
| 25 | 60 | 16.7 | 8.3 | 19.0 |
| 20 | 50 | 9.3 | 10.7 | 11.7 |
| 10 | 80 | 6.7 | 3.3 | 9.8 |
1) What dew point represents
Dew point is the temperature at which air becomes saturated and water vapor begins to condense. When air cools to this point at constant pressure, its relative humidity reaches 100%. Because dew point tracks the absolute moisture content, it is more stable than relative humidity during daily temperature swings.
2) Why it matters in real conditions
Dew point influences comfort, visibility, corrosion, and electrical reliability. Sticky indoor air, foggy mornings, and condensation on pipes all relate to moisture level. Aviation and marine operations monitor dew point to anticipate fog and low ceilings. In electronics, lower dew point reduces moisture-driven leakage and contamination risk.
3) Dew point versus relative humidity
Relative humidity depends on temperature, so a warm afternoon can show a lower percentage than a cool night even if the moisture content is unchanged. Dew point avoids that confusion. For example, air at 30℃ and 50% RH has a dew point near 18℃, while air at 20℃ and 75% RH can have a similar dew point. The moisture load is comparable.
4) Typical ranges and comfort bands
Many forecasters use dew point bands to describe feel. Below 10℃ (50℉) is usually dry. Around 10–16℃ (50–61℉) feels comfortable for most people. At 16–18℃ (61–64℉) humidity becomes noticeable. Around 18–21℃ (64–70℉) feels muggy, and above 21℃ (70℉) can feel oppressive. These are guidelines, not medical thresholds.
5) Condensation and fog risk
Condensation forms when a surface temperature drops below the dew point. A dew point depression (T − Td) near 0–2℃ often signals high fog or dew potential, especially under light wind and clear skies. When Td is close to overnight minimum temperature, morning fog is more likely, reducing visibility and increasing road and runway hazards.
6) HVAC and building science uses
Engineers use dew point to manage condensation on coils, ducts, and chilled-water lines. For indoor air quality, many buildings target a summer indoor dew point near 10–13℃ (50–55℉) to limit mold growth and musty odors while maintaining comfort. In climates with high outdoor dew points, dehumidification load dominates cooling energy use.
7) Field operations and agriculture
Dew point affects evaporation, plant disease pressure, and spray application. High dew points slow drying, increasing leaf wetness duration that favors fungal growth. For outdoor work, rising dew point can indicate heat stress risk even when temperature changes little. Logistics teams also track dew point to prevent condensation inside containers.
8) Accuracy and limitations
This calculator uses a Magnus-type approximation, which is accurate for typical weather ranges but can diverge at extremes and depends on sensor quality. Temperature and RH errors propagate into Td. Use calibrated instruments and consistent placement, avoid direct sunlight, and allow sensors to equilibrate. For high-precision work, consult psychrometric charts or specialized standards.
1) Is dew point the same as humidity?
No. Dew point measures moisture content as a temperature. Relative humidity is a percentage that changes when air temperature changes, even if moisture stays constant.
2) Why does my dew point stay steady while RH changes?
Because RH depends on temperature. If the moisture in air is similar but temperature swings, RH rises at night and falls in daytime, while dew point changes less.
3) What does a small dew point depression mean?
A small difference between air temperature and dew point suggests near-saturation. Condensation, dew, or fog becomes more likely, especially overnight with light winds.
4) When does condensation form on a surface?
Condensation forms when a surface temperature drops below the dew point of surrounding air. Cooling pipes, windows, or metal parts can reach that threshold quickly.
5) Which units should I use for the input?
Use the units you measured. The calculator converts internally and can display results in ℃, ℉, or K, so you can match reporting requirements.
6) Why are there different constant sets?
Magnus-type formulas use fitted constants. Different fits are optimized for slightly different temperature ranges or references, so results can vary by small fractions of a degree.
7) Can dew point be higher than air temperature?
Under normal conditions, no. Dew point equal to air temperature means saturated air. If calculations show Td above T, check inputs, rounding, or sensor accuracy.