Conversion from Terapascals to Hectopascals

Information about the Terapascal (TPa)

The terapascal is a unit of pressure equal to a thousand gigapascals, written TPa. It is ten million bar. Nothing built by engineers operates at this pressure, and the unit belongs instead to two quite separate corners of science: the stiffness of the strongest materials known, and the interiors of large planets.

Carbon nanostructures put it on the map. A single-walled carbon nanotube has a Young's modulus close to one terapascal, and a sheet of graphene the same, which makes them the stiffest materials ever measured relative to their weight. Diamond, long the benchmark, comes in at 1.2 terapascals. Those three numbers are the reason the unit appears at all in materials science.

Stiffness at this level is not the same as strength. A nanotube resists stretching enormously, but a real fibre made of many of them fails at a far lower stress because the tubes slide past one another. Confusing a terapascal modulus with a terapascal breaking strength is one of the commonest errors in popular accounts of these materials.

Planetary interiors reach genuine terapascal pressures. The centre of Jupiter is estimated at three to four terapascals, and the cores of larger gas giants beyond that. Under such conditions hydrogen behaves as a metal, which is what generates the planet's magnetic field, so the unit describes a state of matter rather than a load on a structure.

Laboratories can now reach it briefly. Laser-driven shock compression and pulsed magnetic techniques drive samples into the terapascal range for nanoseconds at a time, long enough to record how a material's density and structure respond. Those experiments are the only direct evidence available about matter under the conditions inside giant planets.

For scale, one terapascal is ten million times atmospheric pressure and about a thousand times the pressure at the centre of the earth divided by three. The number stops being something a person can feel and becomes a description of what atoms do when they are pushed close enough together to change their chemistry.

One terapascal equals 1,000,000,000,000 pascals, 1000 gigapascals, 10,000,000 bar, or about 145 million pounds per square inch.

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.