Definition
The numerical difference, in degrees, between the current air temperature and the dew point temperature at the same location. A small spread indicates the air is near saturation and visible moisture (clouds, fog, or precipitation) is likely; a large spread indicates drier air and a lower likelihood of visible moisture forming.
Plain English
How far apart the air temperature and the dew point are. When the two numbers are close together, the air is almost full of moisture and fog or low clouds become likely. When the numbers are far apart, the air is drier and visibility is usually better.
Context Anchor
Seen in airport weather reports and preflight weather briefings when judging the chance of fog, low clouds, or reduced visibility.
Derivation
Spread' comes from the Old English 'sprædan,' meaning to stretch out or separate. Here it simply means the gap between two values — how far the temperature has 'spread' away from the dew point.
Why Pilots Care
A small spread indicates high humidity and raises the chance of fog, low ceilings, or carburetor icing.
Grounding Statement
If the temperature is 12°C and the dew point is 11°C, the air is very close to forming visible moisture.
Intuition Check
Spread does not mean the weather covers a wide area; here it means the numerical gap between two temperatures. A small spread is not automatically a small concern, because it can mean fog or low clouds are close to forming.
Example Sentence 1
The METAR showed a temperature of 12°C and a dew point of 11°C, so the one-degree spread told the pilot fog was likely to form before sunrise.
Example Sentence 2
Checking the temperature-dew point spread helped the pilot decide whether to delay takeoff due to possible low visibility.