Hectopascal (hPa)
Modern equivalent
100 Pascals = 1 millibar
The SI-conformant name for the millibar, adopted by meteorological services during metrication without altering any recorded value.
When the International System of Units was formalised, meteorologists faced a dilemma: the millibar was not an SI name, but a century of barometric records was written in it. The hectopascal solved the problem elegantly, since one hectopascal is exactly one millibar. Forecasters could adopt the correct SI terminology while every historical chart, shipping forecast and pressure reading remained directly comparable.