1. Atmospheric pressure

For many years, it was customary to express atmospheric pressure in millimeters of mercury. This was obviously due to the fact that this pressure, whether for physical or meteorological applications, was very generally measured using mercury barometers, the directly accessible quantity being the height of the column balancing the atmospheric pressure. However, this approach was somewhat ambiguous, as the pressure thus expressed was not clearly defined, since neither the temperature (and hence the density of the mercury in the column) nor the value of the acceleration of gravity was specified. This led to the definition of standardized measurement conditions, to which reference is made or to which a certain number of corrections can be applied. The reference conditions adopted are as follows: measurements are assumed to be taken in a location where the acceleration of gravity has a so-called...