๐ง Dew Point Calculator
Enter the air temperature and relative humidity to calculate the dew point using the Magnus-Tetens approximation, a formula widely used in meteorology. The dew point is the temperature at which water vapor in the air begins to condense, and it's also a useful gauge of how muggy the air feels.
How to use
- Enter the air temperature (ยฐC).
- Enter the relative humidity (%).
- The dew point is calculated automatically.
How the calculation works
The dew point is the temperature at which air, if cooled, would start condensing water โ the temperature at which relative humidity reaches 100%. This tool uses the Magnus formula common in meteorology: ฮณ = ln(relative humidity รท 100) + 17.27 ร T รท (237.7 + T) Dew point = 237.7 ร ฮณ รท (17.27 โ ฮณ) (T is the temperature in ยฐC) Relative humidity changes with temperature, but the dew point reflects the actual amount of water vapour in the air, so it is better for comparing how muggy it feels. A higher dew point feels stickier.
Worked example
25ยฐC, 50% humidity โ dew point about 13.8ยฐC 30ยฐC, 70% humidity โ about 23.9ยฐC (very muggy) 20ยฐC, 100% humidity โ 20.0ยฐC (the same as the air temperature) When a cold glass is below the dew point, water droplets form on it.
Things to be aware of
- Windows fog up in winter because the glass is colder than the dew point of the indoor air.
- Most people start to find it muggy when the dew point rises above about 20ยฐC.
- The Magnus formula is accurate to within about 0.4ยฐC between roughly โ40ยฐC and 50ยฐC.
FAQ
What is dew point?
It's the temperature air would need to cool to for the water vapor it contains to start condensing. The closer the dew point is to the actual temperature, the more humid the air is.
What's the dew point at 100% relative humidity?
At 100% relative humidity, the dew point exactly equals the air temperature, since the air is already saturated with water vapor.
What formula is used?
The Magnus-Tetens approximation (coefficients a=17.27, b=237.7), a formula widely used in meteorology.