Definition
The temperature to which air must be cooled, at constant pressure and water vapor content, for saturation to occur. When air reaches its dewpoint, water vapor begins to condense into visible moisture such as dew, fog, or cloud.
Plain English
The temperature at which the air becomes so cool that the moisture in it starts turning into water droplets.
Context Anchor
Seen in weather reports, forecasts, and preflight weather planning, usually alongside the current air temperature.
Derivation
From 'dew' (the moisture that forms on cool surfaces overnight) and 'point' (a specific value on a scale). The term literally names the temperature at which dew begins to form.
Why Pilots Care
A small temperature-dewpoint spread signals high humidity and raises the chance of reduced visibility, fog, or carburetor icing.
Analogy
Think of a cold drink on a warm day. The glass sweats because the air touching it has been cooled to its dewpoint, and the moisture in that air condenses on the glass.
Grounding Statement
If the evening air cools down to its dewpoint, moisture may start appearing on grass, windshields, or airplane surfaces.
Intuition Check
Dewpoint is not the same thing as dew, and it is not always the current air temperature. It is the temperature the air would have to reach before moisture begins to condense.
Example Sentence 1
The METAR showed a temperature of 12°C and a dewpoint of 11°C, so the pilot expected fog to form shortly after sunset.
Example Sentence 2
With a 2-degree dewpoint spread and visible moisture, the pilot turned on carburetor heat as a precaution.