Dew Point Calculator
Calculate the dew point temperature from air temperature and relative humidity using the Magnus formula. See also Humidity Calculator and Heat Index Calculator.
What Is Dew Point?
The dew point is the temperature at which air becomes saturated with moisture and water vapor begins to condense into liquid water (dew). When the air temperature drops to the dew point, the relative humidity reaches 100% and moisture condenses on surfaces, forming dew, fog, or frost. The dew point is a more reliable indicator of how humid the air feels than relative humidity alone, because it represents the actual amount of moisture in the air regardless of temperature.
Dew Point Formula (Magnus Approximation)
The Magnus formula provides an accurate approximation of the dew point temperature:
alpha = (a x T) / (b + T) + ln(RH / 100)
Dew Point = (b x alpha) / (a - alpha)
Where:
T = Air temperature in degrees Celsius
RH = Relative humidity (%)
a = 17.27 (constant)
b = 237.7 degrees C (constant)
ln = natural logarithm
Example Calculation
T = 24 degrees C (75 degrees F), RH = 60%
alpha = (17.27 x 24) / (237.7 + 24) + ln(60/100)
alpha = 1.5847 + (-0.5108) = 1.0739
Dew Point = (237.7 x 1.0739) / (17.27 - 1.0739)
Dew Point = 15.8 degrees C (60.4 degrees F)
Dew Point Comfort Scale Diagram
Dew Point vs. Relative Humidity
While both measure moisture, they convey different information. Relative humidity is the percentage of moisture the air holds relative to its maximum capacity at the current temperature — it changes as temperature changes even if moisture stays constant. Dew point is an absolute measure of moisture content — it only changes when actual moisture in the air changes. A dew point of 70 degrees F always feels oppressively humid regardless of the air temperature, while 50% relative humidity can feel dry at 90 degrees F but comfortable at 70 degrees F.
Dew Point Comfort Level Chart
| Dew Point (F) | Dew Point (C) | Comfort Level | Description |
|---|---|---|---|
| Below 50 | Below 10 | Very Dry | Pleasant, may cause dry skin |
| 50-55 | 10-13 | Dry/Comfortable | Ideal outdoor conditions |
| 55-60 | 13-16 | Comfortable | Pleasant for most people |
| 60-65 | 16-18 | Slightly Humid | Becoming noticeable |
| 65-70 | 18-21 | Humid | Uncomfortable for many, muggy |
| 70-75 | 21-24 | Very Humid | Oppressive, difficult to cool down |
| Above 75 | Above 24 | Extremely Humid | Dangerous, heat disorders likely |
How Fog Forms
Fog forms when the air temperature drops to within a few degrees of the dew point, causing water vapor to condense into tiny water droplets suspended in the air. This commonly happens overnight when the ground radiates heat and cools the air above it (radiation fog), or when warm moist air moves over a cold surface (advection fog). If the dew point spread (difference between air temperature and dew point) is less than about 2.5 degrees C (4.5 degrees F), fog formation becomes likely.
Frequently Asked Questions
Can the dew point be higher than the air temperature?
No. The dew point can never exceed the air temperature. When they are equal, the relative humidity is 100% and the air is fully saturated. In practice, dew point is always at or below the current temperature.
What is a comfortable dew point?
Most people find dew points below 60 degrees F (16 degrees C) comfortable. Between 60-65 degrees F it becomes noticeable, and above 70 degrees F (21 degrees C) most people find conditions oppressively humid.
Why is dew point better than humidity for comfort?
Relative humidity is temperature-dependent — 50% RH at 90 degrees F contains much more moisture than 50% RH at 50 degrees F. Dew point directly measures moisture content, so a dew point of 70 degrees F always means the same amount of moisture and the same level of discomfort.
How does dew point affect indoor comfort?
Indoor dew points above 60 degrees F can promote mold growth and dust mites. Air conditioning naturally lowers the dew point by cooling air below its dew point and removing condensed moisture. Ideal indoor dew point is 40-55 degrees F.
What is the dew point spread?
The dew point spread (or depression) is the difference between the air temperature and the dew point. A small spread (under 5 degrees F) indicates high humidity and potential for fog or precipitation. A large spread indicates dry air.
Does dew point change throughout the day?
Dew point typically remains relatively stable throughout the day unless a new air mass moves in. Unlike relative humidity (which rises at night as temperature drops), dew point reflects actual moisture content and changes mainly with weather fronts or wind shifts.