Dew Point Calculator
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The dew point is the temperature at which air becomes saturated and dew or fog forms. At 80 °F and 60% relative humidity the dew point is about 65 °F: muggy. The closer the dew point is to the air temperature, the more humid it feels.
Dew point at the same air temperature for every humidity; your reading is marked.
| Result | |
| Dew point | 64.87 °F (18.26 °C) |
| Relative humidity | 60% |
| Air temperature | 80 °F |
| What it means | |
| How it feels | getting sticky, with muggy evenings |
| Temperature − dew point spread | 15.13 °F |
| Fog or dew likely? | Not at this temperature |
| Estimated cloud base (convective) | about 1,050 m (3,450 ft) above ground |
Show steps
- Magnus formula with Alduchov–Eskridge constants a = 17.625, b = 243.04 °C (accurate to about 0.4 °C from −40 °C to 50 °C).
- Convert to Celsius: T = (80 − 32) × 5/9 = 26.6667 °C.
- \(\gamma = \ln\frac{RH}{100} + \frac{aT}{b + T}\) = ln(0.6) + 1.74263 = 1.23181.
- \(T_{d} = \frac{b\gamma}{a – \gamma}\) = 243.04 × 1.23181 ÷ (17.625 − 1.23181) = 18.2624 °C = 64.8723 °F.
- Comfort scale (based on the dew point in °F): below 55 dry, 55 to 65 sticky, above 65 humid and oppressive.
How to use the dew point calculator
- Choose °F or °C.
- Enter the air temperature and relative humidity (or switch to find humidity from a known dew point).
- Press Calculate to get the dew point, what it feels like and the steps.
The Magnus formula
With T in °C and RH in percent:
\(\gamma = \ln\frac{RH}{100} + \frac{aT}{b + T}, \quad T_{d} = \frac{b\gamma}{a – \gamma}\)
with a = 17.625 and b = 243.04 °C (Alduchov and Eskridge, 1996). It is accurate to within about 0.4 °C between −40 °C and 50 °C.
How the dew point feels
| Dew point | How it feels |
|---|---|
| 55 °F (13 °C) or lower | Dry and comfortable |
| 55 to 65 °F (13 to 18 °C) | Getting sticky, muggy evenings |
| 65 °F (18 °C) or higher | Humid and oppressive |
These bands follow the National Weather Service.
Dew point versus relative humidity
Relative humidity depends on temperature: the same air shows high humidity on a cool morning and lower humidity in the afternoon. The dew point measures the actual moisture, so it is a better guide to how humid it feels. A quick estimate: \(T_{d} \approx T – \frac{100 – RH}{5}\) in °C when RH is above 50%.
Dew point practice quiz: 10 questions
Test yourself with 10 multiple-choice questions. Choose an answer and press Submit to see if you are right and why. Your score appears at the end.
Quiz complete
Frequently asked questions
How do you calculate the dew point?
Use the Magnus formula with the temperature in °C and the relative humidity: γ = ln(RH/100) + 17.625T/(243.04 + T), then Td = 243.04γ/(17.625 − γ).
What is a comfortable dew point?
Most people feel comfortable with a dew point of 55 °F (13 °C) or lower.
What is the difference between dew point and humidity?
Relative humidity is a percentage that changes with temperature. The dew point is a temperature that shows the actual amount of water vapor.
What dew point causes fog?
Fog forms when the air cools to its dew point, so the gap between temperature and dew point shrinks to about zero.
Does the calculator work below freezing?
Yes, down to −60 °C, though below 0 °C the result is the dew point over water; the frost point over ice is slightly higher.
Sources
- National Weather Service: The Dew Point
- Alduchov and Eskridge (1996), Improved Magnus Form Approximation of Saturation Vapor Pressure, Journal of Applied Meteorology
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