Definition
The numerical difference between the current air temperature and the current dew point, usually expressed in degrees Celsius. A small spread indicates the air is near saturation and visible moisture (fog, low clouds, or mist) is likely; a large spread indicates drier air and reduced risk of condensation.
Plain English
How far apart the air temperature and the dew point are. When they are close, the air is almost ready to form fog or low cloud. When they are far apart, the air is drier and visibility is usually better.
Context Anchor
Seen during preflight weather review in airport weather reports, weather briefings, and discussions of fog or low cloud risk.
Derivation
Dew point is the temperature at which water vapor in the air condenses into dew. 'Spread' here simply means the gap between two values. Together the term describes how close the air is to the point where moisture becomes visible.
Why Pilots Care
A small spread signals high humidity and a strong chance of fog or low ceilings forming soon; a large spread means drier air and better visibility.
Analogy
Think of the spread like the remaining room in a cup. When there is very little room left, it does not take much more cooling for moisture to show up as fog or cloud.
Grounding Statement
On a cool, calm morning, if the air temperature falls close to the dew point, tiny water droplets can form near the ground as fog.
Intuition Check
Do not read “spread” as a weather forecast by itself. It is only the gap between two temperatures; a small gap means fog or low cloud is more likely, not guaranteed.
Example Sentence 1
The METAR showed a temperature of 12°C and a dew point of 11°C, so the spread of 1°C made the pilot expect fog by sunrise.
Example Sentence 2
With a 12-degree temperature/dew point spread reported along the route, the pilot expected clear skies and unrestricted visibility.