Dew point
Temperature & humidity dew-point temperature
The dew point is the temperature to which air would need to cool to become saturated with water vapour. It helps explain moisture, misted windows and fog.
What happens as air cools
Water vapour is invisible water in its gaseous state. As moist air cools, its relative humidity rises. At the dew point, it reaches 100 per cent, provided pressure and water-vapour content stay constant during cooling. Further cooling can make vapour condense into liquid droplets.
A glass makes the dew point visible
Suppose a room is at 22 °C with a dew point of 12 °C. A drinking glass cooled to 8 °C may collect droplets on its outside. The water comes from the room air. The glass is not leaking. The whole room does not need to cool to 12 °C; a cold surface is enough.
Reading the weather values
- Temperature close to dew point: Relative humidity is high. Further cooling may favour fog.
- A large gap: The air is relatively dry.
- A high dew point: Plenty of water vapour is present. In warm weather, the air is more likely to feel muggy.
A small gap does not guarantee fog. Wind, cooling and the distribution of moisture matter too. Under ordinary weather conditions, the dew point is no higher than the air temperature.