Definition
The numerical difference, in degrees, between the current air temperature and the current dew point. A small spread indicates the air is close to saturation, making the formation of clouds, fog, or precipitation more likely; a large spread indicates drier air with reduced likelihood of visible moisture.
Plain English
It is just the gap between how warm the air is and the temperature at which that air would become saturated and start producing moisture. The smaller the gap, the closer the air is to making clouds or fog.
Context Anchor
Seen in aviation weather study, preflight weather briefings, and discussions about fog, clouds, visibility, and temperature-dew point relationship.
Derivation
‘Spread’ here simply means ‘the gap between two values.’ It is borrowed from everyday English — the spread between two numbers — and used here to describe the gap between temperature and dew point.
Why Pilots Care
A narrow spread signals high humidity and raises the risk of fog, low ceilings, or reduced visibility during flight operations.
Grounding Statement
If the temperature and dew point are almost the same number, expect moisture in the air — fog, mist, or low clouds. If they are far apart, the air is dry.
Intuition Check
Do not read “spread” as an area or a weather system spreading out. Here it means a simple temperature difference measured in degrees.
Example Sentence 1
The METAR showed a temperature of 12°C and a dew point of 11°C, giving a TDS of just 1°C — fog was almost certain by morning.
Example Sentence 2
During the preflight briefing the instructor noted the temperature dew point spread was narrowing, indicating rising humidity.