Quick answer: At 35 °C and 50% humidity the dew point is 23.0 °C: γ = ln 0.5 + 17.625 × 35 ÷ 278.04 = 1.5254, Td = 243.04 × 1.5254 ÷ (17.625 − 1.5254) = 23.0 °C.
How it's calculated
The calculator uses the Magnus formula with the constants of Alduchov and Eskridge (1996): γ = ln(RH/100) + 17.625T ÷ (243.04 + T), then Td = 243.04γ ÷ (17.625 − γ), with T in °C. It is accurate to within about 0.4 °C from −40 to 50 °C.
The dew point measures the actual amount of moisture in the air, while relative humidity also depends on temperature. That is why a Gulf summer afternoon can show 40% humidity and still feel sticky: at 40 °C, 40% is a dew point of 24 °C.
The US National Weather Service rates a dew point of 55 °F (13 °C) or less as dry and comfortable, 55–65 °F (13–18 °C) as sticky, and 65 °F (18 °C) or more as oppressive.
Worked examples
- 35 °C, 50%: Td = 23.0 °C
- 25 °C, 60%: Td = 16.7 °C
- 40 °C, 40%: Td = 23.8 °C
Frequently asked questions
Can the dew point be higher than the temperature?
No. At 100% humidity the dew point equals the air temperature; it can never be higher.
Why do windows fog up in summer?
An air-conditioned room cools the glass. When the glass is colder than the outdoor dew point, moisture from the humid outside air condenses on it.
Dew point or relative humidity: which tells me more about comfort?
The dew point, because it does not change with the temperature during the day. Relative humidity rises at night and falls in the afternoon even when the moisture in the air stays the same.
What dew point feels comfortable?
Under about 13 °C feels dry and pleasant. Between 13 and 18 °C it starts to feel sticky, and above 18 °C it's oppressive. Gulf coasts in August regularly sit above 25 °C.
What is the dew point at 35 °C and 50% humidity?
About 23 °C (73.5 °F). By US weather service standards that's already oppressive.
When does fog form?
When the air cools down to its dew point, the humidity hits 100% and water condenses into tiny droplets. On Gulf coasts it often happens on winter mornings after a clear, still night.