Definition
The force per unit area exerted by the weight of the column of air above a given point in the atmosphere. At sea level under standard conditions, atmospheric pressure is 29.92 inches of mercury (inHg), or 1013.2 millibars (hPa), and it decreases with increasing altitude.
Plain English
The weight of the air pressing down on everything beneath it. The higher you go, the less air there is above you, so the pressure goes down.
Context Anchor
Seen in weather, altimeter setting, altitude, and aircraft performance discussions.
Derivation
From the Greek atmos meaning vapor or air, and the Latin pressura meaning a pressing. So literally, the pressing of the air — a useful reminder that air has weight and pushes on everything it touches.
Why Pilots Care
It sets the reference for pressure altitude, influences lift and engine power, and must be known to avoid altitude errors.
Analogy
Think of standing at the bottom of a swimming pool. The deeper you are, the more water is above you, and the more pressure you feel. We live at the bottom of an ocean of air — the pressure we feel is the weight of all the air stacked above us.
Grounding Statement
If you climb to a higher altitude, there is less air above the airplane, so atmospheric pressure decreases.
Intuition Check
Atmospheric pressure is not the same as wind. Wind is air moving; atmospheric pressure is air pressing because the air above has weight.
Example Sentence 1
As the aircraft climbed, atmospheric pressure dropped, and the pilot watched the altimeter unwind accordingly.
Example Sentence 2
Lower atmospheric pressure at high elevation airports reduces engine power and requires a longer takeoff roll.