Atmospheric pressure trivia.

[brooksmoses]

It is a source of annoyance that nominal atmospheric pressure at sea level in metric units is about 101 kPa, or 1.01 bar in informal metric units, rather than being equal to a nice round "one bar".

However, today I discovered that -- making allowances for imprecision of topographical maps of Paris and of interpolating on atmosphere-vs-altitude charts -- the nominal atmospheric pressure at the International Bureau of Weights and Measures in Paris is, in fact 100 kPa to at least three digits of precision.

Comments

[redbird]

Also, people talk about "sea level" as if it's the same everywhere, and I was surprised to discovered that it's not. The Pacific is higher than the Atlantic, and the sea level at Bermuda is higher than on the South Carolina coast.

[hitchhiker]

oh neat! if that's the actual historical derivation it would make a nice quiz question.

[brooksmoses]

It would! It's actually complete coincidence, though -- it just randomly happens that with the definitions of meters and kilograms that we inherited from 1790s France, the units of pressure work out such that the nominal atmospheric pressure at sea level is a tiny bit above a 1*10ⁿ multiple of the base pressure unit, and the IBWM happens to be just enough up a hill from the banks of the Seine to drop the 1.01 to 1.00.

Ideally, the IBWM's altitude would be about 30 meters higher to make the coincidence perfect; it's on ground with an altitude of around 65m, and we really want something about 100m. But when
I found out that it was close enough, I decided that it was too good a choice of places to look for somewhere else "significant" that would be a better match.

[brooksmoses]

Indeed, quite! I knew that once, but I'd forgotten it.

[hitchhiker]

hah, yeah, forgot a bar was an SI unit (:

[julesjones]

Thank you for that little bit of geeky joy. :-)