Why was the conversion factor of the metric unit bar chosen the way it was?

Question

The unit bar for pressure is clearly a metric unit, but its order of magnitude is a bit strange. In the centimeter–gram–second system of units we have:

1 bar = 1 000 000 baryes = 1 000 000 dyn/cm²

so the bar is not "coherent" with this system (the factor is not one). Also in the meter–kilogram–second (and SI) system we get:

1 bar = 100 000 pascals = 100 000 N/m²

while in the meter–tonne–second system:

1 bar = 100 pièzes = 100 sn/m²

So my question is simply, where does the conversion factor for bar come from, since it seems to not fit into usual systems? According to the Wikipedia article bar this unit was created already in 1909 by British meteorologist Shaw, but not much detail is provided.

Maybe the factor was simply chosen as the power of ten making the unit closest to the atmospherical pressure at sea level (which is 1.01325 bar by convention, and close to 1.01 bar on average)?

Answer 1

The $\text{pascal}$ seems of a much later date than the $\text{bar}$. In fact, it seems that, at some time, the $\text{bar}$ was adjusted a bit away from the average air pressure on earth (its originally intended definition), to get it "in line" with the SI units, and therefore also with the new or later $1~\text{Pa}=1~{\text{kg}\over\text{m}\cdot\text{s}^2}$.

Meteorologists worldwide have for a long time measured atmospheric pressure in bars, which was originally equivalent to the average air pressure on Earth [...]. After the introduction of SI units, many preferred to preserve the customary pressure figures. Consequently, the bar was redefined as 100,000 pascals, which is only slightly lower than standard air pressure on Earth. [My emphasis.]

^{Pascal (unit)}

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As noted in the comments below, this answer (and, perhaps, the Wikipedia quote) might or might not stand up to scrutiny. Further digging in history seems necessary, but I dug a bit and can't find anything really substantiating this reading. However, I also didn't find anything conclusively and explicitly pointing towards a different reading (i.e., that the magnitude of the $\text{bar}$ is the same now as it was when the $\text{bar}$ was adopted initially).

Answer 2

In 1888, a committee of the British Association proposed the name barad for 1 dyne per cm2. In 1900, a congress of physicists recommended the name 'barye' for 1 dyne per cm2, although the original proposal was to use it for 1 megadyne per cm2. In 1903 Richards and Kennely shortened the unit name to bar. Many meteorologists kept using to the old non-CGS pressure units, mm Hg and atmosphere. Bjerknes was the first influential meteorologist/oceanographer to adopt the CGS system, but he redefined the bar to 1 megadyne per cm2. Consequently, 1 bar was conveniently close to 1 atmosphere.