Definition
The pressure exerted by the weight of the atmosphere at a given location, measured with a barometer and typically expressed in inches of mercury (inHg) or hectopascals (hPa). It varies with altitude and weather conditions and is the value pilots set in the altimeter so it reads correct altitude.
Plain English
It is the weight of the air pressing down at a particular place. That weight changes from day to day and from place to place, which is why pilots reset the altimeter to keep altitude readings accurate.
Context Anchor
Pilots encounter barometric pressure in weather reports, altimeter settings, flight planning, and discussions of changing weather.
Derivation
From Greek baros meaning 'weight' and metron meaning 'measure.' A barometer literally measures the weight of the air, which is exactly what barometric pressure describes.
Why Pilots Care
An incorrect barometric-pressure setting causes the altimeter to show the wrong altitude, leading to terrain or traffic conflicts.
Grounding Statement
At sea level there is more air above you pressing down; as you climb, there is less air above you, so barometric pressure decreases.
Intuition Check
Barometric pressure is not wind. Wind is air moving across the ground; barometric pressure is the air’s pressing force, even when the air feels still.
Example Sentence 1
Before takeoff the pilot set the altimeter to the current barometric pressure given on the ATIS.
Example Sentence 2
A rapid drop in barometric pressure ahead of a cold front required the pilot to update the altimeter setting every twenty minutes.