Definition
A unit of atmospheric pressure used on aviation altimeters and weather reports outside the United States. One hectopascal equals 100 pascals, and 1 hPa is numerically identical to 1 millibar. Standard sea-level pressure is 1013.25 hPa, which corresponds to 29.92 inches of mercury (inHg) used in the U.S.
Plain English
It is the unit most of the world uses to measure air pressure for altimeter settings, instead of the inches of mercury used in the United States.
Context Anchor
Seen in international operations, foreign weather reports, approach charts, and altimeter setting instructions.
Derivation
From 'hecto-' (Greek hekaton, meaning one hundred) and 'pascal' (named after Blaise Pascal, the 17th-century French scientist who studied pressure). So a hectopascal is literally 100 pascals — a convenient size for everyday atmospheric pressure values.
Why Pilots Care
Setting the wrong hPa value produces altitude errors that can result in terrain conflicts or airspace violations.
Grounding Statement
If a controller gives an altimeter setting as 1013 hPa, the pilot needs the altimeter set on the hPa scale, not the inches-of-mercury scale.
Intuition Check
hPa is not a different kind of altimeter setting. It is the same kind of pressure information shown in a different unit.
Example Sentence 1
The London ATIS reported the altimeter setting as 1015 hPa, so the pilot dialed that value into the Kollsman window before descent.
Example Sentence 2
Before crossing into European airspace the crew changed the altimeter from inches of mercury to hPa.