Plain English Meaning

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.

Full 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.

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.