Definition
The numerical difference between the current air temperature and the dew point at a given location and time, expressed in degrees. A small spread indicates that the air is close to saturation; a large spread indicates the air is relatively dry. As the spread narrows toward zero, the likelihood of visible moisture forming — fog, low clouds, or precipitation — increases significantly.
Plain English
The gap between how warm the air is and the temperature at which that air would start forming water droplets. A small gap means moisture is about to show up. A big gap means the air is dry.
Context Anchor
Seen in weather planning when judging the chance of fog, low clouds, or reduced visibility.
Derivation
‘Spread’ here simply means the gap or distance between two values — the same sense as ‘the spread between two prices.’ It is not a technical term; it just labels the numerical difference between temperature and dew point.
Why Pilots Care
A small spread signals higher risk of fog or low ceilings that can affect takeoff, landing, and flight visibility.
Grounding Statement
If the temperature is 15°C and the dew point is 13°C, the spread is 2°C — the air is close to saturation and fog is a real possibility.
Intuition Check
Spread does not mean the temperature and dew point are physically moving apart. Here it means the numerical difference between the two temperatures.
Example Sentence 1
The temperature dew point spread had narrowed to one degree, so the pilot expected fog to form before sunrise and delayed the departure.
Example Sentence 2
A widening temperature dew point spread through the morning cleared the low clouds and improved visibility.