Hectopascal (hPa)

The hectopascal is a unit of pressure equal to one hundred pascals. Its symbol is hPa. It is exactly one millibar, and that identity is the whole reason it exists: when meteorology moved to SI units in the 1980s, the hectopascal let every barometer, chart and forecast keep its numbers unchanged while changing the name on them.

Weather is its home and almost its only use. Standard atmospheric pressure at sea level is 1013.25 hectopascals, a settled high-pressure system reads 1020 to 1035, and a deep Atlantic depression can fall below 950. Isobars on a synoptic chart are drawn at four-hectopascal intervals, so the spacing of the lines is a direct picture of how hard the wind will blow.

Storm intensity is reported the same way. The central pressure of a tropical cyclone is the single most quoted measure of its strength, and the lowest sea-level pressure ever recorded was 870 hectopascals in Typhoon Tip in 1979. A drop of a few tens of hectopascals over a day is enough to turn ordinary weather into a severe event.

Altitude changes the reading steadily. Near sea level pressure falls by roughly one hectopascal for every eight metres of height gained, which is why a barometric altimeter works at all and why a barometer at home responds to being carried upstairs. That gradient weakens with altitude, so the relationship holds only in the lowest few kilometres.

Aviation depends on the unit for safety. Pilots set their altimeters to a local pressure figure given in hectopascals so that everyone in the same airspace measures height from the same datum, and above a defined transition altitude all aircraft switch to the standard setting of 1013 hectopascals so that vertical separation is preserved regardless of the weather. North American practice uses inches of mercury for the same purpose, and the two must never be confused.

For conversion, one hectopascal is one millibar exactly, 0.1 kilopascals, and about 0.0295 inches of mercury. The last of those is the figure to watch, since a reading of 30 inches of mercury and one of 1016 hectopascals describe the same afternoon.

One hectopascal equals 100 pascals, 1 millibar, 0.1 kilopascals, or about 0.0295 inches of mercury.

Hectopascal conversion table