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Inch of mercury (inHg and ″Hg) is a non-SI unit of measurement for pressure. It is used for barometric pressure in weather reports , refrigeration and aviation in the United States . It is the pressure exerted by a column of mercury 1 inch (25.4 mm) in height at the standard acceleration of gravity .
In aviation, pressure altitude is the height above a standard datum plane (SDP), which is a theoretical level where the weight of the atmosphere is 29.921 inches of mercury (1,013.2 mbar; 14.696 psi) as measured by a barometer. [2]
Atmospheric pressure, also known as air pressure or barometric pressure (after the barometer), is the pressure within the atmosphere of Earth.The standard atmosphere (symbol: atm) is a unit of pressure defined as 101,325 Pa (1,013.25 hPa), which is equivalent to 1,013.25 millibars, [1] 760 mm Hg, 29.9212 inches Hg, or 14.696 psi. [2]
This reference can be the mean sea level pressure , the pressure at a nearby surface airport (QFE), or the "standard pressure level" of 1,013.25 hectopascals (29.92 inches of mercury) which gives pressure altitude and is used to maintain one of the standard flight levels.
Within each table, the units are listed alphabetically, and the SI units (base or derived) ... inch of mercury (conventional) inHg ≡ 13 595.1 kg/m 3 × 1 in × g 0:
This is the pressure resulting from a column of mercury of 760 mm in height at 0 °C. For the density of mercury, use ρ Hg = 13,595 kg/m 3 and for gravitational acceleration use g = 9.807 m/s 2. If water were used (instead of mercury) to meet the standard atmospheric pressure, a water column of roughly 10.3 m (33.8 ft) would be needed.
When millimetres of mercury (or inches of mercury) are quoted today, these units are not based on a physical column of mercury; rather, they have been given precise definitions that can be expressed in terms of SI units. [7] One millimetre of mercury is approximately equal to one torr. The water-based units still depend on the density of water ...
Though the lowest pressure may be second to the Night of the big wind low, which saw a value of 27.25 inches of mercury (923 hPa) at Sumburgh Head, Shetland on non-calibrated, non-standard equipment 6–7 January 1839, with the mainland at Cape Wrath reporting an observed pressure of 27.32 inches of mercury (925 hPa). [15]