Dew point is the temperature at which air would become saturated if cooled without changing its water-vapour content. Condensation may begin when air temperature reaches this value.
Unlike relative humidity, dew point is a more direct indication of the actual amount of water vapour. A relative humidity of 70% feels very different at 10°C and 32°C.
Temperature and spread
The temperature–dew-point spread shows how close the air is to saturation. When the spread narrows on a clear, calm night, dew, low cloud or fog becomes more plausible.
It is not a guarantee because wind, cloud cover and the vertical moisture profile matter too.
Humidity and comfort
A high dew point slows sweat evaporation and increases discomfort in hot weather. A low dew point indicates drier air, even when relative humidity rises overnight.
Comfort categories are not medical limits and vary with the person, shade, wind and activity.
Calculation
Dew point can be approximated from temperature and relative humidity using the Magnus relationship. The e‑meteo calculator runs entirely in the browser.
Humidity sensors have uncertainty, especially at very low temperatures or in extremely dry air.
These explanations and calculations are educational. For safety, occupational, health, marine or aviation decisions, follow the appropriate official services.