Earth's gravity measured by NASA
mission, showing deviations from the
of an idealized smooth Earth, the so-called
. Red shows the areas where gravity is stronger than the smooth, standard value, and blue reveals areas where gravity is weaker. (
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 used extensively in the science of
 The gal is defined as 1 centimeter per second squared (1 cm/s2). The milligal (mGal) and microgal (µGal) refer respectively to 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".
 However, use of the gal is deprecated by
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 (see
standard gravity) at its surface is 976 to 983 Gal, the variation being due mainly to differences in
elevation. Mountains and masses of lesser density within the Earth's crust typically cause variations in
gravitational acceleration of tens to hundreds of milligals (mGal). The gravity
gradient (variation with height) above Earth's surface is about 3.1 µGal per centimeter of height (×10−6 s−2), resulting in a maximal difference of about 2 Gal (0.02 m/s 3.12) from the top of
Mount Everest to sea level.
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.