What Is the Dew Point?
The dew point is the temperature to which air must be cooled, at constant pressure and moisture content, for it to become saturated. Cool the air below its dew point and water vapor condenses into dew, fog, or cloud. Unlike relative humidity, which changes as the air warms and cools during the day, the dew point directly reflects how much moisture is in the air, which makes it a better measure of how humid it feels.
The Magnus Formula
This calculator uses the Magnus approximation with the coefficients a = 17.62 and b = 243.12 °C, which is accurate to within a few tenths of a degree for typical weather between about −45 °C and 60 °C.
γ = ln(RH / 100) + a·T / (b + T)
Td = b·γ / (a − γ)
Here T is the air temperature in °C, RH is relative humidity in percent, and Td is the dew point in °C. Reversing the formula gives relative humidity from temperature and dew point:
RH = 100 × exp( a·Td / (b + Td) − a·T / (b + T) )
Worked Example
Air at 25 °C (77 °F) with 60% relative humidity:
- γ = ln(0.60) + 17.62 × 25 / (243.12 + 25) = −0.5108 + 1.6429 = 1.1321
- Td = 243.12 × 1.1321 / (17.62 − 1.1321) = 16.7 °C (62.0 °F)
A dew point near 62 °F is starting to feel a little sticky for most people.
Dew Point Comfort Guide
- Below 50 °F (10 °C): dry, comfortable
- 50–59 °F (10–15 °C): comfortable
- 60–64 °F (16–18 °C): becoming humid, somewhat sticky
- 65–69 °F (18–21 °C): humid and uncomfortable for many
- 70–74 °F (21–24 °C): very humid, oppressive
- 75 °F (24 °C) and above: extremely humid, heat stress risk
These bands are a general guide; individual comfort varies with temperature, wind, sun, and activity.
Frequently Asked Questions
How do you calculate dew point from temperature and humidity?
Use the Magnus formula: γ = ln(RH/100) + 17.62·T/(243.12 + T), then dew point = 243.12·γ/(17.62 − γ), with temperatures in °C. This calculator does it for you in °F or °C.
What is a comfortable dew point?
Most people feel comfortable when the dew point is below about 60 °F (16 °C). Above 65 °F (18 °C) the air feels humid, and above 70 °F (21 °C) it feels oppressive.
What is the difference between dew point and relative humidity?
Relative humidity is the amount of moisture compared with the maximum the air can hold at its current temperature, so it changes as temperature changes. Dew point measures the actual moisture content, so it is a steadier indicator of how muggy it feels.
Can the dew point be higher than the air temperature?
No. When the air temperature falls to the dew point, the air is saturated (100% relative humidity) and condensation forms. The dew point can equal the temperature but not exceed it.
What is the frost point?
When the dew point is below freezing, moisture deposits as frost instead of dew. The true frost point is slightly higher than the dew point calculated over water; this calculator uses the over-water formula.