Dew Point Calculator
Dew point, temperature spread, and comfort reading from air temp and humidity.
What it does
Dew point names the moisture actually in the air: cool the air to it and dew forms. Relative humidity misleads because warm air holds more, so 50% at 80 degrees feels far muggier than 100% at 30 degrees.
The tool above takes air temperature with relative humidity in either system. It returns the dew point with the spread below air temperature and a comfort reading beside them.
The default of 77 F at 60% gives a 62.1 F dew point, 14.9 F below air: sticky hints. A hot 30 C at 70% gives 23.9 C: oppressive moisture by the same single formula.
The definition follows Weather Service glossary practice with field office comfort guidance matching its bands. Both place dry comfort at or under 55 F dew point and oppressive air at 65 F and above.
How to use
- Pick imperial (F) or metric (C).
- Enter the current air temperature.
- Enter relative humidity as a percent.
- Read dew point first for the moisture verdict.
- Read spread for fog and dew chances overnight.
Formula
- T
- Air temperature converted to C inside.
- RH
- Relative humidity percent.
- spread
- Air minus dew point: small means fog or dew near.
Fahrenheit converts to C first (minus 32 times 5/9), then one path runs, so systems agree. At 100% humidity the math returns air temperature exactly with zero spread.
Worked example
Example: 77 F with 60% humidity
Imperial defaults: air 77 F (25 C), humidity 60%.
- Celsius: (77 - 32) x 5/9 = 25 C.
- y = ln(0.6) + 17.625 x 25 / 268.04 = 1.1329.
- Dew point: 243.04 x 1.1329 / 16.4921 = 16.7 C (62.1 F).
- Spread: 25 - 16.7 = 8.3 C (14.9 F) below air.
- Tool shows 62.1 F, 14.9 F below air, sticky comfort.
Reference table
| Air | Humidity | Dew point |
|---|---|---|
| 77 F | 60% | 62.1 F |
| 25 C | 60% | 16.7 C |
| 30 C | 70% | 23.9 C |
Limits
- Near ground air assumed; mountain and desert layers vary.
- Sensors in sun or near vents bias both inputs.
- Frost forms below freezing instead of dew at the same point.
- Indoor comfort adds clothing and airflow the number misses.
Common mistakes
- Judging mugginess by humidity percent alone.
- Expecting dew point above air temperature.
- Feeding heat index or wind chill as air temperature.
- Reading car sensors parked in sun.
- Ignoring overnight cooling toward the point.
Try it yourself
Quick checks. Answers sit tucked under each question.
30 C at 70% humidity?
23.9 C dew point, 6.1 C below air: oppressive moisture.
100% humidity at 20 C?
Dew point 20 C with zero spread: saturated air.
Small spread overnight?
Fog and dew likely as air cools those last degrees.
Questions
What is dew point?
The temperature air must cool to for saturation and dew, at constant moisture.
What dew point feels comfortable?
At or under 55 F feels dry; 55 to 65 turns sticky; 65 plus feels oppressive.
Dew point or humidity?
Dew point for comfort. Humidity percent swings with temperature while moisture stays put.
Can dew point pass air temperature?
No. It caps at air temperature, which reads as 100% humidity.
Why morning fog?
Night cooling reaches the point, so vapor condenses near the ground.
Dew versus frost?
Same point, different phase: dew above freezing, frost crystals below it.