How this works
Dew point is the temperature to which air must cool — at constant pressure — for water vapour to start condensing into dew. It's an absolute measure of moisture in the air, unlike relative humidity which depends on the current temperature. A 60 °F dew point at 60 °F air = 100% RH (foggy), but the same 60 °F dew point at 90 °F air = ~36% RH (dry-feeling).
Meteorologists prefer dew point over relative humidity for judging comfort because the body responds to the absolute moisture, not the percentage. The bands most people use: under 13 °C / 55 °F is dry, 13–18 °C / 55–65 °F is comfortable, 18–21 °C / 65–70 °F starts to feel sticky, 21–24 °C / 70–75 °F is oppressive, and above 24 °C / 75 °F is miserable — sweat barely evaporates and exertion becomes risky. Daily highs above 27 °C / 80 °F dew point (Florida summer, parts of monsoon Asia) are physiologically dangerous for sustained outdoor work.
Dew point also predicts overnight fog: when surface air cools to or below the dew point, fog forms. It tells you the maximum AC load on a humid day — air conditioning has to remove moisture, and the energy required scales with how far you have to drop the temperature below the dew point. The calculator uses the Magnus-Tetens approximation, accurate to within 0.4 °C across normal weather ranges.
The formula
T = current air temperature (°C), RH = relative humidity (%). The formula assumes air pressure ≈ 1013 hPa (sea level); altitude variation is small enough to ignore for practical purposes.
Example calculation
- Air 28 °C, RH 65%.
- γ = ln(0.65) + (17.625 × 28) / (243.04 + 28) ≈ 1.387; T_dew = (243.04 × 1.387) / (17.625 − 1.387) ≈ 20.8 °C — sticky, near oppressive.
Frequently asked questions
Why is dew point a better comfort indicator than humidity?
Relative humidity is a percentage of saturation at the current temperature, so 70% RH at 32 °C feels very different from 70% RH at 18 °C — the warm-day air actually contains far more moisture. Dew point cuts the temperature dependence: a 21 °C dew point feels sticky whether the air is 24 °C or 32 °C. Local TV weather increasingly leads with dew point in summer for this reason.
Can dew point be higher than air temperature?
No — dew point is by definition ≤ air temperature, with equality at 100% relative humidity (saturated, fog/mist forming). If a sensor reports a dew point above the temperature, it's a calibration error or the temperature has just dropped below dew point and condensation is actively forming. Quick test: if humidity ≥ 100%, your humidity sensor probably needs recalibration.
How does dew point relate to mould risk indoors?
Mould needs surfaces that stay above the dew point for any length of time — when a wall is colder than the dew point, water condenses on it and feeds mould growth. In winter, the worst spots are external walls behind furniture (poor air circulation, surface several °C below room temp). Aim for an indoor dew point under 10 °C / 50 °F in heating season; ventilate after showers and cooking; insulate cold-bridge points.