Definition
A paired weather value showing the current air temperature and the dewpoint temperature at a given location. Temperature is the actual temperature of the air. Dewpoint is the temperature to which that air must be cooled, at constant pressure, for it to become saturated and produce condensation (visible moisture such as dew, fog, or cloud). The two values are usually reported together because the gap between them indicates how close the air is to saturation.
Plain English
Two numbers shown side by side: how warm the air is right now, and how cool it would have to get before moisture starts forming. When the two numbers are close, the air is nearly ready to produce fog or low clouds.
Context Anchor
Pilots see TEMP / DEWPOINT in METARs, ATIS, AWOS or ASOS broadcasts, weather briefings, and cockpit weather displays.
Derivation
Dewpoint is literally the point (temperature) at which dew forms. The word comes from the everyday observation that grass becomes wet overnight when air cools enough for water vapour to condense out of it.
Why Pilots Care
The spread between the two values shows how close the air is to saturation, directly affecting decisions about fog, low ceilings, and icing potential.
Grounding Statement
If the air temperature drops to the dewpoint, moisture comes out of the air as fog, cloud, or dew.
Intuition Check
Do not read TEMP / DEWPOINT as two versions of the same temperature. TEMP is the air temperature now; DEWPOINT is the lower temperature at which that same air may start forming visible moisture.
Example Sentence 1
The METAR showed 14/13, so the pilot expected fog to form shortly after sunset as the temperature dropped toward the dewpoint.
Example Sentence 2
A wide TEMP / DEWPOINT spread told the crew that fog was unlikely during the night IFR flight.