Dorothy Parker once remarked of an acquaintance that she was fluent in 27 languages, and unable to say "No" in any of them. While it does not do to take the analogy too far, it is true to say that meteorologists require a similar degree of conceptual versatility when it comes to coping with the great variety of units used for describing atmospheric pressure.
The Earth can be thought of as supporting the weight of its overlying atmosphere. The pressure exerted at any point on the Earth's surface is the weight of the entire column of air, right up to the top of the atmosphere, standing on unit area at that point. It varies from place to place; the average value of atmospheric pressure, to choose just one of the many units by which it is described, is about 14.7lb per square inch.
The mercury barometer was invented in the 17th century by an Italian called Evangelista Torricelli, and consisted of a glass bulb fitted with a neck "two cubits" long, or about 40 inches. The tube was filled with mercury and inverted into a dish containing more mercury. Torricelli found that in these circumstances the mercury in the tube fell to a certain level - about 30 inches above the surface of the mercury in the dish - but no further. Moreover, the length of the column rose and fell with changes in the weather.
Now it was primarily with this latter quality that early users of barometers were preoccupied, rather than with the niceties of how to label atmospheric pressure. It seemed natural to capture the relationship between the weather and the mercury in terms of the length of the column, either in inches or in millimetres. In due course aneroid barometers were invented which measured pressure without the use of any liquid whatsoever, but the unit of equivalent length of mercury was retained.
By the late nineteenth century, however, this anomaly had become an irritant, and in 1900 meteorologists adopted the millibar - one thousandth of a bar, the unit in which pressure was then measured. And then in 1986 there was a further change, which brought the hectopascal - it being 100 Pascals, the basic unit of pressure in the now official Systeme International.
But plus ca change, plus c'est la meme chose. . Millibars and hectopascals are numerically equivalent, so 1000 mbs is exactly 1000 hPa. And to convert either of them to inches of mercury, it is necessary only to divide by 33.86 - which, if you work it out, means that 1000 millibars is the same pressure as 29.53 inches.