Definition
A unit of atmospheric pressure used in most countries outside the United States, equal to 100 pascals. One hectopascal (hPa) equals one millibar (mb), so standard sea level pressure of 1013.25 mb is also 1013.25 hPa. Altimeter settings outside the U.S. are typically issued in hectopascals rather than inches of mercury.
Plain English
A measurement of air pressure used internationally. The number is the same as millibars, just under a different name. When flying outside the U.S., controllers will give you the altimeter setting in hectopascals instead of inches of mercury.
Context Anchor
Seen when flying outside the United States, especially in weather information and altimeter-setting instructions.
Derivation
From 'hecto-' meaning one hundred, and 'pascal,' the SI unit of pressure named after French scientist Blaise Pascal. So a hectopascal is literally 100 pascals. The pascal alone is too small a unit for practical weather use, so the hectopascal became the working unit for atmospheric pressure.
Why Pilots Care
Incorrect conversion between hectopascals and inches of mercury produces altitude errors that can lead to controlled flight into terrain or airspace violations.
Intuition Check
Hectopascals are not a different kind of weather report; they are a different unit for the same kind of pressure information. The key is knowing which unit the country or procedure is using.
Example Sentence 1
Approaching Frankfurt, the controller issued the altimeter setting as 'QNH 1019 hectopascals,' and the pilot set 1019 in the hPa window of the altimeter.
Example Sentence 2
Before crossing into Canadian airspace the crew reset the altimeter from 29.92 to the local hectopascal value.