Calculate the dew point temperature from air temperature and relative humidity using the Magnus formula.
| Dew Point | Comfort Level |
|---|---|
| < 10°C (50°F) | Dry / Comfortable |
| 10–15°C (50–60°F) | Comfortable |
| 16–18°C (60–65°F) | Slightly Humid |
| 18–21°C (65–70°F) | Humid |
| 21–24°C (70–75°F) | Very Humid |
| ≥ 24°C (75°F) | Extremely Oppressive |
| Temp \ RH | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|
| 10°C | -7 | -3 | 0 | 3 | 5 | 7 | 8 |
| 15°C | -2 | 2 | 5 | 7 | 10 | 12 | 13 |
| 20°C | 2 | 6 | 9 | 12 | 14 | 16 | 18 |
| 25°C | 6 | 10 | 14 | 17 | 19 | 21 | 23 |
| 30°C | 11 | 15 | 18 | 21 | 24 | 26 | 28 |
| 35°C | 15 | 19 | 23 | 26 | 29 | 31 | 33 |
| 40°C | 19 | 24 | 28 | 31 | 33 | 36 | 38 |
A 72°F room at 50% humidity has a dew point of 11.3°C, or 52.3°F, which is comfortable air. Type the same 72 with the unit menu left on °C and the page returns 56.7°C, a number that looks official and means nothing. The menu defaults to °C, so check it before you read the answer.
Walk through the page's own defaults: 30°C and 65% relative humidity.
The page uses the Magnus formula with a = 17.625 and b = 243.04. First it finds alpha = ln(RH ÷ 100) + a·T ÷ (b + T). Here ln(0.65) = −0.4308 and 17.625 × 30 ÷ 273.04 = 1.9365, so alpha = 1.5057.
Then dew point = b × alpha ÷ (a − alpha) = 243.04 × 1.5057 ÷ (17.625 − 1.5057) = 365.96 ÷ 16.119 = 22.70°C. In Fahrenheit that is 22.70 × 9 ÷ 5 + 32 = 72.9°F.
The page also reports absolute humidity. Saturation vapor pressure is 6.112 × e^(17.62T ÷ (243.12 + T)) = 42.34 hPa, and AH = 216.7 × (0.65 × 42.34) ÷ (273.15 + 30) = 19.67 g/m³. A dew point between 21 and 24°C is labeled Very Humid / Oppressive.
Housekeeping: Fahrenheit entries are converted with (T − 32) × 5 ÷ 9 first, and humidity is held between 1 and 100, because zero would make the logarithm undefined. Results show one decimal.
Relative humidity depends on temperature. At 60%, the dew point is 7.3°C at 15°C air, 16.7°C at 25°C and 26.1°C at 35°C. The warmer air holds far more moisture at the same percentage.
Usually, yes. 32°C at 40% and 18°C at 90% have almost the same dew point, about 16.5°C, and feel similar in how sticky the air is, though the humidity figures are very different.
Compare the dew point with the temperature of the surface. Room air at 21°C and 60% has a dew point of 12.9°C, so a window pane at 8°C or a cold water pipe will collect moisture.
It runs, but below 0°C the formula is for dew over supercooled water, and the true frost point is slightly different. At −5°C and 80% the page gives −7.9°C.
No. They are a rule of thumb. People and regions differ, and the bands in the table are meant for general guidance only.