Dew Point Calculator
What is the Dew Point?
The dew point is the specific temperature at which air must be cooled for water vapor to condense into liquid water (dew). In simpler terms, it is the temperature at which the air becomes 100% saturated with moisture. When the air temperature cools to its dew point, clouds, fog, or dew on the ground will begin to form.
Why Does Dew Point Matter?
Unlike relative humidity, which changes based on the air temperature, the dew point provides an absolute measure of how much moisture is in the air. This makes it a critical metric for several fields:
- Meteorology: Forecasters use dew point to predict the severity of thunderstorms and the likelihood of fog.
- HVAC & Home Comfort: High dew points (above 65°F / 18°C) make the air feel "sticky" or "muggy" because sweat cannot evaporate from your skin effectively.
- Industrial Painting: Professionals must ensure the surface temperature is at least 5°F above the dew point to prevent moisture from being trapped under the paint.
If the outdoor temperature is 80°F (26.7°C) and the relative humidity is 50%, the dew point is approximately 59.4°F (15.2°C). At this level, most people find the air comfortable. However, if the humidity rises to 75% at the same temperature, the dew point jumps to 71°F, making the air feel very humid.
The Magnus Formula
This calculator uses the Magnus-Tetens approximation, which is widely recognized for its accuracy in Earth's typical temperature range. The formula is:
γ(T, RH) = ln(RH/100) + (bT / (c + T))
Tdew = (c * γ) / (b – γ)
Where b = 17.27 and c = 237.7.
Dew Point Comfort Scale
| Dew Point (°F) | Comfort Level |
|---|---|
| Below 50°F | Very Dry / Pleasant |
| 50°F to 60°F | Comfortable |
| 60°F to 65°F | Noticeable Humidity |
| 65°F to 70°F | Sticky / Uncomfortable |
| 70°F to 75°F | Oppressive |
| Over 75°F | Extremely Dangerous |