Table of Contents
What is Dew Point?
The dew point temperature is the temperature at which air becomes saturated with water vapor and condensation begins. When air cools to its dew point, water vapor condenses into liquid droplets, forming dew, fog, or clouds. The dew point is a direct measure of atmospheric moisture content and is more meaningful than relative humidity for expressing how muggy or comfortable the air feels.
Unlike relative humidity, which changes with temperature even if moisture content stays the same, the dew point remains constant as long as the actual amount of water vapor in the air does not change. This makes it a more reliable indicator for weather forecasting, HVAC design, and comfort assessment. Meteorologists and building scientists rely heavily on dew point calculations for predicting condensation and managing indoor air quality.
Magnus Approximation
Where a = 17.27, b = 237.7°C, T is air temperature in °C, and RH is relative humidity (%). This approximation is accurate to within 0.4°C for typical atmospheric conditions.
Dew Point Comfort Scale
| Dew Point | Comfort Level |
|---|---|
| Below 10°C | Dry, pleasant |
| 10-16°C | Comfortable |
| 16-18°C | Slightly humid |
| 18-21°C | Humid, uncomfortable |
| Above 21°C | Oppressive |
Temperature and Humidity Reference
| Temp (°C) | 50% RH Dew Pt | 80% RH Dew Pt |
|---|---|---|
| 20 | 9.3°C | 16.4°C |
| 25 | 13.9°C | 21.3°C |
| 30 | 18.4°C | 26.2°C |
| 35 | 23.0°C | 31.1°C |
Frequently Asked Questions
Why is dew point better than relative humidity?
Relative humidity is relative to the current temperature. Air at 30°C with 50% RH feels much more humid than air at 10°C with 50% RH because the warmer air holds far more moisture. The dew point tells you the actual moisture content directly. A dew point above 20°C always feels oppressive regardless of the air temperature.
What causes condensation on windows?
Window condensation occurs when the glass surface temperature drops below the indoor dew point. This commonly happens in winter when warm, humid indoor air contacts cold window surfaces. The solution is either reducing indoor humidity or improving window insulation (double or triple glazing) to keep the glass surface above the dew point.
How does dew point affect HVAC design?
HVAC systems must cool air below its dew point to remove moisture (dehumidification). The greater the difference between the supply air temperature and the dew point, the more moisture is removed. In humid climates, dehumidification often requires more energy than sensible cooling, making dew point a critical design parameter for air conditioning systems.