I think no one will object to the statement that at an altitude of two kilometers the air is thinner and the atmospheric pressure is less than at the sea level.
If we put these words in a scientific form, it turns out that the pressure (density) of the gas depends on its altitude in a gravitational field. On this phenomenon method of barometric leveling is built.
Barometric leveling - method of determining the height difference between two points by atmospheric pressure measured at these points .Since the atmospheric pressure, in addition to the altitude above the sea level is also dependent on the weather, for example, on the water vapor content of air, if it is possible to the method is applied to make measurements at points with the smallest interval between the measurements, and the points themselves should not be located too far from each other.
The difference in altitude is calculated as follows.
There is a rather complicated formula of Laplace:
It is, in addition to temperature and pressure also takes into account the absolute humidity and latitude of the measuring point, that is, in practice it seems to be not in use.
And use a simple Babinet formula
where - Gas expansion factor equal to
Indeed, in an era without computers and calculators, even this formula was ... well, not difficult but hard for calculations, so to determine the difference in height people used auxiliary barometric level tables.
Barometric stage - the height at which we must ascend so the pressure drop by 1 mm Hg
That is, we took and simplified Babinet formula to expression
Thus, by measuring, for example, the pressure difference at the average temperature t and the average pressure p, Meteorologist could find the value of the barometric stage from the table, and multiply it by the amount of pressure difference.
It is clear that the formula gives the result with a margin error, but at the same time it is approved that the error does not exceed 0.1 - 0.5% of the measured altitude.
The method of barometric leveling allows to determine the height of a point above sea level, without resorting to the geodetic leveling.
In practice, the height of the point above sea level is determined by using the closest ranging mark, height above sea level of which is known.
For example, the ranging mark156 meters. Barometer shows that ranging mark at 748 mmHg, being transferred to the defined point, barometer shows 751 mmHg. The average temperature is 15 degrees Celsius. Using the Babinet formula obtain -33.78 m, i.e. the point below the ranging mark at 33.78 meters and it's height is approximately 122.22 m. Taking the average pressure of 748 mmHg and using barometric table we get -33.85,i.e. the height is approximately 122.15 m.
The calculator below illustrates everything said above.