Definition
The pressures exerted by the weight of the atmosphere at a given location and altitude, typically measured in inches of mercury (in Hg) or hectopascals (hPa). Barometric pressure decreases as altitude increases, and varies with weather systems at any given altitude.
Plain English
How heavily the air is pressing down at a particular place and height. The higher you go, the less air is above you, so the pressure drops.
Context Anchor
Seen in high-altitude flight, cabin pressure, altimeter setting, weather, and altitude-induced decompression sickness discussions.
Derivation
From the Greek 'baros' meaning 'weight,' combined with 'metric' (measure). A barometer literally measures the weight of the atmosphere pressing down on a surface.
Why Pilots Care
Rapid drops in barometric pressure during ascent allow nitrogen to leave solution in the blood and joints, producing the painful and potentially disabling symptoms of decompression sickness.
Grounding Statement
At sea level on a standard day, the air above you weighs about 14.7 pounds per square inch; climb to 18,000 feet and roughly half that air is now below you, so the pressure on your body and instruments is cut in half.
Intuition Check
Do not think of barometric pressure only as a weather-report number. In this context, it means the actual air pressure around the pilot’s body, and that pressure changes with altitude and cabin pressure.
Example Sentence 1
As the pilot climbed through 10,000 feet, the falling barometric pressure required a careful watch on cabin altitude and oxygen use.
Example Sentence 2
Before a high-altitude flight the instructor reviewed how falling barometric pressures affect gas solubility in tissue.