Definition
The difference, in degrees, between the current air temperature and the current dewpoint temperature. As the spread narrows toward zero, the air is approaching saturation and visible moisture (fog, low cloud, or mist) becomes increasingly likely.
Plain English
How far apart the air temperature and the dewpoint are. When those two numbers get close together, the air is nearly full of moisture and fog or low cloud is likely to form.
Context Anchor
Seen in weather reports, forecasts, and fog discussions when deciding how close the air is to forming fog or low clouds.
Derivation
‘Spread’ here simply means the gap between two values. It is borrowed from everyday usage — the spread between two numbers — and applied to the temperature and dewpoint readings.
Why Pilots Care
A narrowing spread signals rising humidity and the likelihood of fog forming, which can suddenly reduce visibility during takeoff, approach, or landing.
Grounding Statement
If the temperature is 12°C and the dewpoint is 11°C, the spread is 1°C — the air is almost saturated and fog is a real possibility, especially as the air cools overnight.
Intuition Check
Spread does not mean the size of a weather area here. It means the numerical difference between air temperature and dew point.
Example Sentence 1
With a temperature/dewpoint spread of only 1°C and the sun setting, the pilot expected fog to form within the hour.
Example Sentence 2
The crew checked the spread on the METAR before departure to judge the chance of low ceilings developing en route.