What the dew point tells you
The dew point is the temperature to which air must cool for its water vapor to condense into dew, fog or droplets on a cold glass. Unlike relative humidity, which changes with temperature during the day, the dew point measures the actual amount of moisture in the air. That makes it the best single number for how muggy it feels: meteorologists use it to forecast fog and frost, and it tells you when a window, a wall or a cold-water pipe will start to sweat.
The formulas
The calculator uses the Magnus approximation, accurate to about 0.1 °C between −40 and 50 °C:
- γ = ln(RH ÷ 100) + 17.62 × T ÷ (243.12 + T)
- Dew point = 243.12 × γ ÷ (17.62 − γ), with T in °C
It also gives the absolute humidity (grams of water per cubic meter of air) and the heat index, the “feels like” temperature published by the National Weather Service, computed from the temperature in °F and the humidity.
Worked example: 77 °F and 50% humidity
- 77 °F = 25 °C.
- γ = ln(0.5) + 17.62 × 25 ÷ 268.12 = −0.693 + 1.643 = 0.950.
- Dew point = 243.12 × 0.950 ÷ (17.62 − 0.950) = 13.9 °C, or 57 °F: comfortable.
- The air holds 11.5 g of water per cubic meter, and the heat index is about 77 °F: at this humidity it feels like the real temperature.
How the dew point feels
| Dew point | Feeling |
|---|---|
| Below 50 °F (10 °C) | Dry, crisp |
| 50–60 °F (10–16 °C) | Comfortable |
| 60–70 °F (16–21 °C) | Humid, sticky |
| Above 70 °F (21 °C) | Oppressive, typical of a Gulf Coast summer |
Dew point at 77 °F for other humidity levels
| Relative humidity | Dew point | Absolute humidity |
|---|---|---|
| 30% | 43 °F (6.2 °C) | 6.9 g/m³ |
| 50% | 57 °F (13.9 °C) | 11.5 g/m³ |
| 70% | 66 °F (19.1 °C) | 16.1 g/m³ |
| 90% | 74 °F (23.2 °C) | 20.7 g/m³ |
| 100% | 77 °F (25 °C) | 23.0 g/m³ |
At 100% humidity, the dew point equals the air temperature: the air is saturated.
Practical uses
- Condensation and mold: any surface colder than the dew point gets wet. If your basement is at 68 °F and 60% humidity, the dew point is about 54 °F, so a cold-water pipe at 50 °F will drip. A dehumidifier lowers the dew point below that temperature.
- Painting and coatings: most paints should not be applied when the surface is less than 5 °F above the dew point.
- Fog and frost: when the evening dew point is close to the forecast low, expect fog; if it is below freezing, frost.
- Heat safety: the heat index rises quickly above a 70 °F dew point, because sweat stops evaporating. Above a heat index of 103 °F, the National Weather Service warns of a danger of heat exhaustion.