Isobars - Why are they called Hectopascals?

By Bob McDavitt - MetService Weather Ambassador

Barometer

That Barometer you may have in the hallway is a wonderful instrument for following the weather.

In an article issued last year we looked at how to forecast the weather using a barometer.
In this article we examine why the units of air pressure are called hectoPascals.

Baros is the Greek word for “weight� and metron is the Greek word for “measure�, but the invention of the barometer wasn’t done in Greece and actually marked the end the some of the famous Grecian Aristotle’s ideas.

France, mid seventeenth century

Blaise Pascal was born in France in June 1623, the son of Etienne Pascal, a lawyer. His mother died when he was three. He was educated by his dad and proved to be a child prodigy in anything mathematical.

Almost 20 years later in 1642 Galileo died - and one of Galileo’s students, Evangelista Torricelli, was working with the first-ever documented barometer and wrote a paper in 1643 describing his experiments on how air pressure could support a column of water 32 feet high, and the top of this column would go up and down with the weather (but that’s another story and can wait for another article).

Around this time Etinenne and Blaise belonged to a book club in Paris. This club also had amongst its members a French monk, Marin Mersenne. In 1644 Mersenne visited Italy on a trip to Rome and in an idea-gathering mission he also visited Torricelli. He returned to Paris with a choice collection of writings for the book club, including Torricelli’s paper. Thus it was that in 1646 Blaise got to read this article and started thinking about air pressure.

Theoretical Disagreements

Blaise spent a few years reproducing Torricelli’s experiments and trying out different liquids such as water versus red wine. In 1647 he wrote some papers on the properties of the void at the top of barometer (Expériences nouvelles touchant le vide). Then he and Rene Descartes have some long chats about this topic. Descartes could not believe that a void or vacuum existed or that a barometer was really measuring air pressure. He supported the explanation by Greek philosopher Aristotle that this space was filled with a substance called aether, which was extremely tenuous and able to flow through tiny pores in the glass tube and fill in the space left by the receding water. The aether was also used by Aristotelians to explain how light from far-away stars reaches us by travelling through a substance. Things came to a pretty pass.

Scientific experimental evidence

Blaise was a scientist who understood the power of experimental evidence and so he then built a portable barometer using a glass tube of mercury inverted into a dish of mercury. Because he had weak knees he asked his brother-in-law, Florin Périer, to do the hard bit - carry this device up the Puy de Dôme, a conspicuous mountain in Rouen familiar to Blaise from his childhood, and take a series of measurements. Sure enough, the height of the supported mercury column got less with increasing altitude. The amount of fluid in the tube was responding to changes in air pressure. So it was air pressure that was responsible for the height of the column, rather than the size of any void left over at the top. This experiment was carried out in September 1648 and verified Evangelista Torricelli’s original hypothesis about atmospheric pressure on the equilibrium of liquids.

(Above) Le Puy de Dôme in central France, site of famous PASCAL experiment, click here to see source image.

The Puy de Dôme experiment became famous in the world of science for finally putting an end to the Aristotelians aether theory, and the principle it proved became known as Pascal’s Law. More details about this are available here .

Pascal’s barometers quickly became the best tool for measuring altitude, proving accurate enough to give good measures of depths for example in coal mines. It wasn’t long before barometers were also adopted by sea captains to help show the dropping air pressure that precedes wet windy weather. To this very day, even though we measure temperature, wind, rainfall, and humidity as well as pressure—it is the PRESSURE LINES that we draw on weather maps that are accepted as capturing and showing the weather pattern.

Click here for the latest weather pattern.

Isobars used to be measured in millibars (and one millibar does equal one hectoPascal), but a millibar is based on the old (pre 1960) CGS unit of pressure called a Bar (a million dynes per square centimeter.
When the MKS system of units was drawn up in 1960, as a commemoration of Pascal’s pressure experiment, PASCAL was chosen for the name for a unit of pressure (a Newton per square metre). And so, isobars are now labeled in hectoPascals. The prefix of hecto is from the Greek word hekaton for “hundred�. So 1010hPa means 101,000 Pascals or 101,000 Newtons per square metre- that’s about a tonne per square metre. Yes, that’s the pressure exerted by about 32 feet or 10 metres of water.

Pascal on the 500 Franc French Bank note: click here to see source image.

Bob McDavitt, 4 March 2008