What does dewpoint mean?
The dewpoint is the temperature at which air becomes saturated with water vapour: cool it further at constant pressure, and moisture begins to condense into dew, fog, or frost. Unlike relative humidity, which changes with temperature, the dewpoint reflects the actual amount of moisture in the air, making it one of the most reliable indicators of atmospheric humidity. Meteorologists use it constantly — a high dewpoint signals muggy, uncomfortable conditions, while a small gap between air temperature and dewpoint warns of fog or cloud formation. The term appears most often in weather forecasting and aviation, though it also matters in HVAC design, agriculture, and any field concerned with condensation. Commonly written either as one word ("dewpoint") or two ("dew point"), it derives from the Old English "dēaw," meaning dew, combined with "point," the specific temperature threshold at which saturation occurs. Understanding it offers an intuitive grasp of why some humid days feel far worse than others.
nounThe temperature at which air becomes saturated with water vapour and dew begins to condense; below this temperature, cooling causes moisture to deposit as dew, fog, or frost.
- The temperature at which air, when cooled at constant pressure and constant water-vapour content, reaches saturation and condensation begins — the key measure of atmospheric humidity in weather forecasting.
"The forecast warned that the dewpoint would climb into the seventies, making the evening feel oppressively muggy."
"With the dewpoint hovering near sixty degrees, the morning grass was soaked before sunrise."
"Pilots check the spread between air temperature and dewpoint to gauge the risk of fog forming along their route."
The plural is rare because dewpoint is usually discussed as a single measured value for a given air mass; plurals appear mainly when comparing values across locations, times, or scenarios.
"The study compared afternoon dewpoints across coastal and inland stations over a ten-year period."
When the dewpoint hits the mid-70s °F, even a short walk can feel like wading through soup — meteorologists call anything above that 'oppressive,' and your sweat agrees.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of dewpoint
The term combines "dew," from Old English "dēaw" (cognate with German "Tau" and Dutch " dauw"), with "point," from Latin "punctum," meaning a small hole or prick, later a precise spot or moment. As a compound, it emerged in the nineteenth century alongside the development of modern thermodynamics and meteorology, when scientists such as John Dalton and Heinrich Wilhelm Dove formalized the physics of condensation. Its cognates lie not in other words for dewpoint itself, but in the broader family of "dew" derivatives like "dewy" and "dewdrop."
How dewpoint is actually used
The word is frequently written as two words ("dew point") as well as closed up ("dewpoint"); both spellings are widely accepted in technical and popular usage. It is primarily a technical and meteorological term but appears commonly in everyday weather reporting, especially in American forecasts where dewpoint values in degrees Fahrenheit are quoted directly as a comfort indicator.
Easily confused with dewpoint
"Due point" is not a standard English term; it is a misspelling arising from the identical pronunciation of "due" and "dew," whereas the correct meteorological term is always spelled with "dew."