Dew Point Calculator
Find dew point from temperature and relative humidity with Magnus-Tetens formula and comfort level.
Inputs
Dew Point (°C)
18.4
Temperature at which the air becomes saturated and condensation forms.
Dew Point (°F)
65.1
Temperature–Dew Point Spread (°C)
11.6
A spread below 2.5°C indicates fog or heavy condensation risk.
Comfort Level
Very humid — uncomfortable, oppressive.
Step by step
Inputs
= T = 30°C (86°F), RH = 50%
Compute γ = ln(RH/100) + a·T/(b+T)
ln(50/100) + 17.625×30 / (243.04 + 30)
= 1.243382
Dew point = b·γ / (a − γ)
243.04 × 1.243382 / (17.625 − 1.243382)
= 18.4°C (65.1°F)
Temperature–dew point spread
30°C − 18.4°C
= 11.6°C (< 2.5°C indicates near-saturation / fog risk)
Comfort level
= Very humid — uncomfortable, oppressive.
How it works
The dew point is the temperature to which air must be cooled (at constant pressure and humidity) for water vapour to condense into liquid water. It is a direct measure of the absolute moisture content of the air — unlike relative humidity, which changes with temperature even when the actual moisture content stays the same. The Magnus-Tetens formula gives a fast and accurate approximation using only temperature and relative humidity. Dew points above 18°C feel muggy to most people; above 21°C is oppressive; above 24°C is dangerous. A dew point near the air temperature (small temperature–dew point spread) signals near-saturation and fog risk.
Formula
Magnus-Tetens dew point
γ = ln(RH/100) + (a × T) / (b + T); dew point = b × γ / (a − γ)
- T
- Air temperature in °C
- RH
- Relative humidity as a percentage
- a
- Magnus constant 17.625
- b
- Magnus constant 243.04 °C
- Td
- Dew point temperature in °C
- γ
- Intermediate Magnus-Tetens term
Frequently Asked Questions
Why is dew point a better measure of humidity comfort than relative humidity?
Relative humidity depends on temperature: the same amount of water vapour gives 100% RH at the dew point but only 40% RH at a much warmer temperature. On a hot day, 50% RH can feel very muggy, while 50% RH on a cold day is comfortable. Dew point is invariant to temperature and directly represents how much moisture is in the air, making it a more stable comfort metric.
At what dew point does it start to feel uncomfortable?
Dew points below 10°C feel dry and comfortable. From 13–16°C is noticeably humid. Above 18°C most people begin to feel oppressed; above 21°C is genuinely uncomfortable; above 24°C is considered dangerously tropical. The exact threshold varies with acclimatisation.
What does a small temperature–dew point spread mean?
A spread less than 2.5°C (4.5°F) means the air is very close to saturation. This is the condition for fog formation, heavy morning dew, and condensation on cool surfaces. A spread of 0 means the air is exactly at saturation (relative humidity = 100%).
Can the dew point ever exceed the air temperature?
No — by definition the dew point cannot be higher than the air temperature. If you observe the dew point ≥ temperature in a weather report, it means relative humidity ≈ 100% and the sensor has a small calibration error.