Conversion from 100 Hectopascals to Micropascals

Information about the 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.

Information about the Micropascal (µPa)

The micropascal is a unit of pressure equal to one millionth of a pascal. Its symbol is µPa. It is one of the smallest pressure units in practical use, and it owes its place to a single number: 20 micropascals, the reference pressure against which every sound level in air is measured.

That figure is the threshold of hearing — roughly the quietest sound a healthy young ear can detect at a frequency of about 1,000 hertz. When acousticians defined the decibel scale for airborne sound they needed a fixed pressure to compare against, and they chose that threshold. Zero decibels means a sound pressure of 20 micropascals; every decibel figure quoted for a road, a machine or a concert is a ratio to that number.

The scale that follows is steep. Twenty decibels is ten times the reference pressure, 200 micropascals. Sixty decibels, ordinary conversation, is a thousand times it, or 20 millipascals. A hundred and twenty decibels, the threshold of pain, is a million times it: 20 pascals. The ear covers a range of a million to one in pressure, and the micropascal sits at the bottom of it.

Underwater acoustics uses the same unit but a different reference. Sound in water is referred to 1 micropascal rather than 20, because the threshold of human hearing is meaningless in the sea. That difference matters enormously: a level quoted in decibels underwater is not comparable to one in air, and the gap between the two conventions is about 26 decibels before any other correction. Sonar figures, whale-song measurements and shipping-noise studies all carry the re 1 µPa qualifier for that reason.

Outside acoustics the micropascal appears in vacuum science. A good high vacuum is around 100 micropascals, and ultra-high vacuum, the regime used for surface physics and particle accelerators, runs from a few micropascals down to nanopascals. Pumping a chamber to that level takes hours of baking to drive adsorbed gas off the walls.

The unit also shows up in radiation pressure and in the very small pressure differences that laboratory instruments resolve. Anything measured in micropascals is a measurement rather than a force anyone would feel, which is exactly what makes the reference-level convention so useful.

One micropascal equals 0.000001 pascals, 0.001 millipascals, 1,000 nanopascals, or about 0.000000000145 pounds per square inch.