Inch of mercury
From Glossary of Meteorology
inch of mercury
A common unit used in the measurement of atmospheric pressure.
1) One inch of mercury (in Hg) is defined as that pressure exerted by a 1-in. column of mercury at standard gravity and a temperature of 0°C:
This is a unit recommended for meteorological use.
2) One 45° inch of mercury [in Hg (45°)] is defined as that pressure exerted by a 1-in. column of mercury at 45° latitude at sea level and a temperature of 0°C.
It is evident that for most purposes these two units are interchangeable. When this is not the case, the unit should be carefully specified. Metric, rather than the English, units of length are used in many branches of science, and in other parts of the world. The early development and continued widespread use of the mercury barometer has fostered this manner of expressing atmospheric pressure. Although it has largely been replaced by the hectopascal (hPa) in most meteorological work, inches of mercury is still used in altimetry, and it remains the most common form of barometer scale calibration.