Dew Point Calculator
Compute dew point from temperature and humidity
How to Use
- Enter the current air temperature
- Select your preferred temperature unit (°C or °F)
- Enter the relative humidity percentage (0.1-100%)
- Click calculate to find the dew point temperature
- Review the approximate dew point in °C and °F
What is Dew Point?
Dew point is the temperature at which water vapor in the air begins to condense into liquid water at constant atmospheric pressure. When the air temperature drops to the dew point, dew, frost, or fog can form.
Unlike relative humidity, dew point is an absolute measure of moisture in the air. It's a more accurate indicator of how humid it actually feels and is widely used in weather forecasting and HVAC applications.
Dew Point vs. Relative Humidity
While both measure atmospheric moisture, they differ significantly:
| Characteristic | Relative Humidity | Dew Point |
|---|---|---|
| Definition | Ratio of actual to maximum moisture | Temperature at saturation occurs |
| Units | Percentage (%) | Temperature (°C/°F) |
| Dependence | Varies with temperature | Independent of temperature |
| Comfort indicator | Less reliable | More reliable |
Dew Point Comfort Levels
Dew point is an excellent indicator of human comfort:
- Below 10°C (50°F): Very dry, comfortable for most people
- 10-16°C (50-60°F): Comfortable, pleasant humidity
- 16-20°C (60-68°F): Getting humid, still comfortable for many
- 20-24°C (68-75°F): Uncomfortable, muggy feeling
- 24-26°C (75-79°F): Very uncomfortable, oppressive
- Above 26°C (79°F): Extremely uncomfortable, dangerous for prolonged exposure
Practical Applications
Dew point calculations are essential in many fields:
- Weather forecasting: Predicting fog, dew, and frost formation
- HVAC systems: Designing efficient heating and cooling
- Agriculture: Determining optimal conditions for crop growth
- Aviation: Assessing icing conditions and visibility
- Construction: Preventing moisture damage in buildings
- Storage: Protecting sensitive materials from condensation
The Magnus-Tetens Formula
This calculator applies the Magnus approximation over liquid water to air temperatures from -45°C to 60°C (-49°F to 140°F). Dew points below -45°C extend the formula beyond its stated range:
γ(T,RH) = (a·T)/(b+T) + ln(RH/100)
Td = (b·γ)/(a−γ)
Here a = 17.62 and b = 243.12°C for all supported air temperatures, including below 0°C. This is a liquid-water dew point, not a frost point over ice.
Source: World Meteorological Organization, Guide to Meteorological Instruments and Methods of Observation (WMO-No. 8), Volume I, Annex 4.B: https://repository.oceanbestpractices.org/bitstream/handle/11329/83/wmo_8_en-2012.pdf?sequence=1
Frequently Asked Questions
- What's the difference between dew point and frost point?
- Frost point is the temperature at which water vapor is in equilibrium with ice. This calculator estimates dew point over liquid water even below 0°C (32°F); it does not calculate frost point.
- Why does dew point feel more accurate than relative humidity?
- Relative humidity changes with temperature, so 50% humidity at 30°C feels much more humid than 50% at 15°C. Dew point represents the actual moisture content regardless of temperature.
- Can dew point be higher than air temperature?
- No, dew point cannot exceed the air temperature. When dew point equals air temperature, the air is saturated (100% relative humidity) and condensation occurs.
- What does it mean when dew point and temperature are close together?
- When dew point and air temperature are within a few degrees, the air is near saturation. This indicates high humidity, poor visibility potential, and comfortable conditions for fog formation.
- How accurate is the Magnus-Tetens formula?
- This is an approximation based on the Magnus formula for liquid water. The WMO lists its water formula for -45°C to 60°C, but the result also depends on the accuracy of the temperature and humidity inputs. No fixed dew-point error is guaranteed, especially when the computed dew point falls below -45°C.
- Why is dew point important for HVAC systems?
- HVAC systems must manage both temperature and humidity. Knowing the dew point helps prevent condensation in ducts, ensure proper dehumidification, and maintain indoor comfort efficiently.