How the Dew Point Calculator Works
Air always holds some water as an invisible gas, called water vapor. Warm air can hold more than cold air. The dew point is the temperature to which the air must cool, with no change in its water, until it is full. At that temperature the vapor starts to turn into liquid water, as dew on the grass, mist, or fog. The Dew Point Calculator finds it from the air temperature and the relative humidity.
What you enter
- The air temperature, in degrees Celsius or Fahrenheit.
- The relative humidity, the percent of the water that the air could hold at this temperature. It must be above 0% and up to 100%.
The formula
The calculator uses the Magnus formula, a well-known approximation for the pressure of water vapor:
alpha = (17.27 x T) / (237.7 + T) + ln(RH / 100) dew point = 237.7 x alpha / (17.27 - alpha)
T is the temperature in degrees Celsius, RH is the relative humidity in percent, and ln is the natural logarithm. A temperature in Fahrenheit is changed to Celsius first, and the answer is shown in both. The formula is accurate to about half a degree between 0 and 60 degrees Celsius, and it can be used from -45 degrees.
Worked example: 25 °C and 60% humidity
| Step | Calculation | Result |
|---|---|---|
| Temperature term | 17.27 x 25 / (237.7 + 25) | 1.6436 |
| Humidity term | ln(0.60) | -0.5108 |
| alpha | 1.6436 - 0.5108 | 1.1328 |
| Dew point | 237.7 x 1.1328 / (17.27 - 1.1328) | 16.7 °C (62.1 °F) |
| Air minus dew point | 25 - 16.7 | 8.3 °C |
- Step: Temperature term
Calculation: 17.27 x 25 / (237.7 + 25)
Result: 1.6436 - Step: Humidity term
Calculation: ln(0.60)
Result: -0.5108 - Step: alpha
Calculation: 1.6436 - 0.5108
Result: 1.1328 - Step: Dew point
Calculation: 237.7 x 1.1328 / (17.27 - 1.1328)
Result: 16.7 °C (62.1 °F) - Step: Air minus dew point
Calculation: 25 - 16.7
Result: 8.3 °C
If the air cooled down to 16.7 °C, for example at night, dew would start to form.
The comfort scale
The dew point tells you how humid the air feels better than the relative humidity does, because it does not change with the temperature. One common guide:
| Dew point | How it feels |
|---|---|
| Below 10 °C (50 °F) | Dry |
| 10 to 16 °C (50 to 60 °F) | Comfortable |
| 16 to 18 °C (60 to 65 °F) | A little humid |
| 18 to 21 °C (65 to 70 °F) | Sticky |
| 21 to 24 °C (70 to 75 °F) | Very humid |
| 24 °C (75 °F) or more | Oppressive |
Each block of the chart is 1 °C, from -10 °C to 30 °C. The calculator draws the same scale with a marker at your dew point.
-10 °C 30 °C ████████████████████████████████████████
Colors: blue is dry, green is comfortable, light green is a little humid, yellow is sticky, orange is very humid and red is oppressive.
Table: the dew point (°C)
| Air | 30% | 50% | 70% | 90% |
|---|---|---|---|---|
| 10 °C | -6.8 | 0.1 | 4.8 | 8.4 |
| 15 °C | -2.4 | 4.7 | 9.6 | 13.4 |
| 20 °C | 1.9 | 9.3 | 14.4 | 18.3 |
| 25 °C | 6.2 | 13.8 | 19.1 | 23.2 |
| 30 °C | 10.5 | 18.4 | 23.9 | 28.2 |
| 35 °C | 14.8 | 23.0 | 28.7 | 33.1 |
Chart: the dew point at 25 °C for different humidity
The drier the air, the lower the dew point. The bars start at -10 °C, and each block is 0.9 °C.
20% ████████████ 0.5 °C 40% ███████████████████████ 10.4 °C 60% ██████████████████████████████ 16.7 °C 80% ████████████████████████████████████ 21.3 °C 100% ████████████████████████████████████████ 25.0 °C
Chart: the dew point at 50% humidity
With the same relative humidity, warmer air has a much higher dew point. Each block is 1 °C from -10 °C.
0 °C █ -9.2 °C 10 °C ██████████ 0.1 °C 20 °C ███████████████████ 9.3 °C 30 °C ████████████████████████████ 18.4 °C 40 °C ██████████████████████████████████████ 27.6 °C
What the dew point tells you
- Dew, mist and fog: when the air is cooled to its dew point, water condenses. The closer the air is to its dew point (a difference of less than about 2.5 °C), the more likely fog is.
- Frost: if the dew point is below 0 °C, the water turns directly to frost when the air cools.
- Condensation on windows and pipes: a cold surface that is colder than the dew point of the room gets wet.
- Comfort: a high dew point means that sweat dries slowly, so the air feels muggy, even if the temperature is not very high.
- Mold and damp: a high dew point indoors, for a long time, helps mold to grow.
What this calculator does not include
- The effect of the air pressure and the height above sea level, which are small for the dew point.
- Humidity below 0 °C with respect to ice (the frost point).
- Temperatures outside -45 °C to 60 °C.
This tool gives an estimate for general information. The comfort levels are a common guide, and people feel the air differently.