Definitions
| Relative humidity (RH) | = (actual water vapor) / (saturation) × 100% |
|---|---|
| Dew point | Temp at which air becomes saturated and water condenses |
| Wet-bulb | Temp measured by a thermometer with wet wick — close to dew point at high RH |
| Absolute humidity | g of water vapor per m³ of air |
Dew point vs comfort
| Dew point (°C) | Dew point (°F) | Comfort |
|---|---|---|
| < 10 | < 50 | Very dry / comfortable |
| 10–16 | 50–60 | Comfortable |
| 16–18 | 60–65 | Slightly humid |
| 18–21 | 65–70 | Noticeably humid |
| 21–24 | 70–75 | Uncomfortable |
| > 24 | > 75 | Oppressive |
RH at 20 °C (air temp)
| Dew point (°C) | Relative humidity |
|---|---|
| 0 | ~27% |
| 5 | ~39% |
| 10 | ~55% |
| 15 | ~75% |
| 18 | ~89% |
| 20 | 100% (saturated) |
Practical thresholds
- Condensation on windows: indoor dew point > interior glass temperature.
- Mold growth: sustained RH > 70% on cold surfaces.
- ESD risk: RH < 30% dramatically raises static-discharge risk.
- HVAC target: 40–50% RH indoor in summer, 30–40% in winter (to avoid condensation on windows).
- Magnus formula: T_d ≈ (b·γ(T,RH)) / (a − γ(T,RH)) where γ = ln(RH/100) + aT/(b+T), a=17.625, b=243.04 °C.
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