Home / Topics / Heat & Thermodynamics
Humidity and dew point
Beginner
Moisture content of air and condensation temperature.
Relative humidity is the ratio of water vapour pressure to saturated vapour pressure. The dew point is the temperature at which air becomes saturated and condenses.
At 25 °C and 60% RH, dew point ≈ 17 °C.
Cool surfaces below the dew point get wet.
In more detail
Relative humidity is the ratio of the water vapour pressure in air to the saturation vapour pressure at the same temperature. The dew point is the temperature at which the air becomes saturated.
Real-world uses
- Weather forecasts report humidity and dew point for comfort and fog prediction.
- Greenhouses and museums control humidity to protect plants and objects.
- Meteorologists use the dew point to predict fog and dew, since the air cools until it becomes saturated.
Open the interactive lesson, with simulation and quiz →
Frequently asked questions
What is dew point?
The temperature at which water vapour condenses when air is cooled at constant pressure.
Why does humid air feel hotter?
It slows evaporation from the skin, reducing cooling.
What is the dew point?
The temperature at which the air becomes saturated with water vapour, so that condensation starts when the air is cooled to it.
Related topics in Heat & Thermodynamics
Joule–Thomson effect · Heat death of the universe · Arrow of time · Greenhouse effect · Thermal equilibrium in the Sun-Earth system · Thermal conductivity of materials