Definition
The pressure exerted by the vapor above a liquid that is evaporating in a closed container. A liquid with a high vapor pressure evaporates easily at low temperatures, while a liquid with a low vapor pressure requires more heat before it begins to vaporize.
Plain English
How readily a liquid wants to turn into a gas. The easier it evaporates, the higher its vapor pressure.
Context Anchor
Seen in aviation maintenance discussions of fuels, solvents, oils, and other fluids, especially when temperature affects how easily a fluid gives off vapor.
Derivation
From Latin vapor (steam, mist) and pressure (a pressing force). Together: the pushing force exerted by the gas form of a liquid as it tries to escape into the air above it.
Why Pilots Care
Aviation fuels are blended with vapor pressure in mind. A fuel that vaporizes too easily can cause vapor lock in fuel lines or pumps; a fuel that vaporizes too poorly can be hard to start in cold weather.
Analogy
A warm closed fuel container builds more vapor push inside than a cold one, much like a warm soda bottle feels more pressurized before you open it.
Grounding Statement
Picture a sealed jar half-full of gasoline. Some of the liquid evaporates into the space above it, and that vapor pushes against the walls of the jar. That push is vapor pressure.
Intuition Check
Do not read “vapor pressure” as just “how much vapor is present.” It means the pressure created by that vapor under a specific temperature condition.
Example Sentence 1
Aviation gasoline is formulated with a controlled vapor pressure so it ignites reliably without vaporizing too readily in the fuel lines.
Example Sentence 2
Maintenance crews verify the vapor pressure rating of new avgas before it is added to the aircraft.