Gal (unit)

File:Southern ocean gravity hg.png

Gravity anomalies covering the Southern Ocean are shown here in false-color relief. Amplitudes range between −30 mGal (magenta) to +30 mGal (red). This image has been normalized to remove variation due to differences in latitude.

The gal (symbol: Gal), sometimes called galileo after Galileo Galilei, is a unit of acceleration typically used in precision gravimetry.<ref>Barry N. Taylor, Guide for the Use of the International System of Units (SI), 1995, NIST Special Publication 811, Appendix B.</ref><ref name="SIbrochure">BIPM SI brochure, 8th ed. 2006, Table 9: Non-SI units associated with the CGS and the CGS-Gaussian system of units Template:Webarchive.</ref><ref>Some sources, such as the University of North Carolina Template:Webarchive, the European Space Agency, and ConversionTables.com Template:Webarchive state that the unit name is "galileo". The NIST and the BIPM are here considered as more authoritative sources regarding the proper unit name.</ref> The gal is defined as 1 centimeter per second squared (1 cm/s²). The milligal (mGal) and microgal (μGal) are respectively one thousandth and one millionth of a gal.

The gal is not part of the International System of Units (known by its French-language initials "SI"). In 1978 the CIPM decided that it was permissible to use the gal "with the SI until the CIPM considers that [its] use is no longer necessary".<ref name="SIbrochure"/><ref>NIST Guide to SI Units; Section 5, Units Outside the SI; Subsection 5.2: Units temporarily accepted for use with the SI.</ref> Use of the gal was deprecated by the standard ISO 80000-3:2006, now superseded.

The gal is a derived unit, defined in terms of the centimeter–gram–second (CGS) base unit of length, the centimeter, and the second, which is the base unit of time in both the CGS and the modern SI system. In SI base units, 1 Gal is equal to 0.01 m/s².

The acceleration due to Earth's gravity at its surface is 976 to 983 Gal, the variation being due mainly to differences in latitude and elevation. Standard gravity is 980.665 Gal. Mountains and masses of lesser density within the Earth's crust typically cause variations in gravitational acceleration of 10 to hundreds of milligals (mGal).

The gradient of gravity is the gravity gradient, usually measured in eotvos (0.1 μGal/m). The vertical gravity gradient near Earth's surface is ~3.1 μGal per centimeter of height (Template:Val), resulting in a maximal difference of about 2 Gal (0.02 m/s²) from the top of Mount Everest to sea level.<ref>Gravity Measurements Template:Webarchive. University of Calgary. Retrieved November 21, 2009.</ref>

Unless it is being used at the beginning of a sentence or in paragraph or section titles, the unit name gal is properly spelled with a lowercase g. As with the torr and its symbol, the unit name (gal) and its symbol (Gal) are spelled identically except that the latter is capitalized.

Examples

Examples of Gal<ref name=":0">Template:Cite journal</ref>

Effect type	Effect size (Gal)
Various places on the surface of earth	976–983
Standard gravity	980.665
Surface gravity of moon	161–164
Change between Mount Everest's peak to sea level	~2
Precision of Kater's pendulum	Template:Val
Magma entry under Mount Etna in 2002 October	Template:Val
Semidiurnal and diurnal earth tide, due to sun and moon	Template:Val
Pole tide component of earth tide, due to Chandler wobble	Template:Val
Precision of a superconducting gravimeter	Template:Val
Background level of the free oscillations of earth ("earth hum")	Template:Val<ref name=":1">Template:Cite journal</ref>
Theoretical precision of an optomechanical gravimeter	Template:Val<ref>Template:Cite journal</ref>

Many geophysical effects produce variation in surface gravity on the order of 0.1 to 1 μGal. These include change in ground water level by ~0.1 m, underground magma formations near a volcano, daily evapotranspiration from a deciduous forest, yearly change in ground height due to subsidence, the free oscillations of earth excited by major earthquakes, etc. For example, the maxima precision of a superconducting gravimeter is sufficient to measure groundwater level change of 1 mm, and can detect the onset of the 2011 Tohoku earthquake 510 km away from the epicenter.<ref name=":0" />

By combining data from many measurements, the sensitivity of gravimetry can be decreased further. 100 days of measurement with a superconducting gravimeter reached Template:Val in precision, which was sufficient to detect the hum of the earth.<ref name=":1" />

See also

• Earth's gravity
• Eotvos (unit)
• g-force (g)
• Gravimetry
• Gravity
• Gravitational constant (G)
• Gravitational field
• Gravity gradiometry