Humidity and dew point: how to read them
Relative humidity is the best-known figure, but the dew point tells you better how much water is in the air. This guide explains both and how to use them to tell whether it will be muggy, foggy or frosty. It is general information.
Relative humidity
It is the percentage of water vapor in the air compared with the maximum it could hold at that temperature. It changes a lot during the day: it is usually higher before dawn, when it is colder, and lower in the afternoon, even if the amount of water in the air is the same.
Dew point
It is the temperature at which the air becomes saturated and vapor starts to condense. Unlike relative humidity, it shows how much moisture is really in the air, regardless of the time of day.
How it feels (approximate values)
- Dew point below about 10 °C: dry, pleasant air.
- Between about 10 and 16 °C: comfortable.
- Between about 16 and 20 °C: starts to feel humid and sticky.
- Above about 20 °C: muggy, especially when it is hot.
Fog, dew and frost
- When the air temperature falls to the dew point, vapor condenses: dew appears on surfaces, or fog in the air.
- If the dew point is below 0 °C, vapor freezes directly and frost forms.
- That is why clear, calm nights, when the temperature drops a lot, favor fog and frost.
Where to see them on this site
Under “More data” on each city's weather page you will find humidity and dew point. They are model estimates for the point and may differ from what you measure where you are.
General, educational information: it does not replace professional advice or official warnings.