DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

Merian’s equation
Merian’s equation An equation for determination
of the natural periods of a basin of water,
such as a lake or harbor. The natural period is
also referred to as the seiching period.
meridian A great circle passing through the
poles of a sphere; in astronomy the meridian of
an observer is the meridian passing through the
zenith (i.e., directly overhead).
meridional In the direction of the meridians,
the lines of constant longitude (i.e., northsouth).
See also zonal.
meromixis Refers to a natural water body,
which does not experience deep convection
(such as due to cooling) on a regular basis (e.g.,
annual). Meromixis is mostly linked to permanent
salinity gradients at greater depth than occurs
in isolated ocean basins (such as in the
Canada Basin) as well as in many deep lakes
(such as Baikal, Tanganyika, Malawi, etc.), as
well as in eutrophic lakes.
Merope Magnitude 4.2 type B5 star at RA
03 h 46 m , dec +23 ◦ 56 ′ ; one of the “seven sisters”
of the Pleiades.
mesogranulation Intermediate scale of convection
on the sun. Mesogranules are typically
5,000 to 10,000 km in size with speeds (vertical
or horizontal) of ∼60ms −1 .
meson An unstable bosonic massive particle
which undergoes strong nuclear interactions.
mesoscale A motion scale in the atmosphere
smaller than a synoptic scale like the scale of
a weather system, but larger than a microscale
like the scale of an individual fair weather cumulus
clouds. Mesoscale phenomena include
the weather fronts, tornadoes, and storm and
maintain waves. In the ocean, the scale of
the Gulf Stream eddies or rings is considered
a mesoscale.
mesosphere In atmospheric physics, the atmospheric
layer which lies between the stratosphere
and the thermosphere. It is bounded on
the bottom by the stratopause and at the top by
the mesopause. On Earth, the mesosphere lies
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between 50 and 85 km altitude. The mesosphere
is characterized by a decrease in temperature
with altitude. The transport of radiation within
the terrestrial mesosphere is primarily due to the
absorption and emission of heat by carbon dioxide.
The mesosphere and the stratosphere combined
are often referred to as the middle atmosphere.
In solid earth geophysics, the lower portion
of the upper mantle to the lower mantle underlying
the asthenosphere. The Earth’s crust and
mantle can be divided into three different portions,
namely, lithosphere, asthenosphere, and
mesosphere, in terms of their depth range and
dynamic properties. The mesosphere has difficulty
flowing compared to the asthenosphere.
Since motion of the mesosphere is considered
to be much slower than plate motions near the
Earth’s surface, absolute plate motion is understood
as relative motion to the mesosphere.
mesotrophic water Water with moderate
concentrations of phytoplankton biomass;
chlorophyll a concentration ranges from between
approximately 0.5 to 10 mg m −3 .
Messier The Messier catalog is a list of nebulous
objects in the sky compiled by Charles
Joseph Messier (1730–1817). Messier was a
comet hunter and kept a list of fuzzy objects in
the sky which were not comets, starting with
the Crab Nebula. There are 103 objects in the
catalog and each is indicated by the letter “M”
followed by its number in the list. The catalog
includes galaxies (e.g., M31, the Andromeda
Galaxy), planetary nebulae (e.g., M42, the Great
Nebula in Orion), and star clusters (e.g., M13,
the Great Cluster in Hercules).
metallic hydrogen When hydrogen molecules
are placed under sufficiently high pressure,
the molecules dissociate and the resultant
atomic phase has electrons that are free to
move about the lattice, giving the solid metallic
properties. The exact pressure of the transition
depends on the temperature, but shock experiments
indicate a transition pressure of 140 GPa
at 3000 K.
metallicity The abundance, in stars or galaxies,
of all elements heavier than hydrogen and