DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

eam spread function
erates auroral electrons downwards, possibly a
quasi-neutral electric field associated with upwards
Birkeland currents. See conics.
beam spread function The irradiance distribution
on the inner surface of a sphere as generated
by an initially collimated, narrow beam
and normalized to the beam power [m −2 ]; numerically
equals the point spread function.
beam transmissometer An instrument for
measuring the fraction of a collimated beam lost
by absorption and scattering per unit distance of
photon travel; measures the beam attenuation
coefficient.
Beaufort wind scale A descriptive table of
wind speeds developed by Admiral Sir Francis
Beaufort in 1806. For practical use, in particular
at sea, wind speeds are described by the
states of the sea surface and the wave height
H . The original scale considers 12 grades from
calm to hurricane; in 1956, grade 12 was divided
into grades from 12 to 17. The wind velocity is
related to the Beaufort Force approximated by
v = B 1.5 , where v is the wind speed measured
at 10 m height and B is the Beaufort Force. See
tables on pages 39 and 40.
bed load A term used in the study of sediment
transport by moving fluids. Bed load denotes
the fraction of sediment transported very
close to the bed, such as the particles that bounce
along the bed. The remainder of the sediment is
suspended in the water column and is referred
to as suspended load and wash load.
Beer’s law Radiation traveling in a certain
direction in a scattering or absorbing medium is
exponentially attenuated.
Belinda Moon of Uranus also designated
UXIV. Discovered by Voyager 2 in 1986, it is
a small, irregular body, approximately 34 km in
radius. Its orbit has an eccentricity of 0, an inclination
of 0 ◦ , a precession of 129 ◦ yr −1 , and a
semimajor axis of 7.53 × 10 4 km. Its surface is
very dark, with a geometric albedo of less than
0.1. Its mass has not been measured. It orbits
Uranus once every 0.624 Earth days.
© 2001 by CRC Press LLC
Benard cell When a gas or plasma is heated
uniformly from below, convection takes place
in vertical cells, the Benard cells. The motion
is upward in the center of the cell and downward
at the cell boundaries. In laboratory experiments,
the pattern of convection cells is regular
and long-lived. Natural Benard cells can
be observed in the terrestrial atmosphere under
calm conditions or as granulation at the top of
the solar convection zone.
Benioff zone In a subduction zone the
oceanic lithosphere descends into the mantle at
velocities of about 10 cm per year. The upper
boundary of the descending plate is a fault zone
between the plate and overlying crustal and mantle
wedge. The earthquakes on this fault define
the Benioff zone. The zone has a typical dip
of 30 ◦ to 45 ◦ and extends to depths of 670 km.
The existence of these dipping zones of seismicity
was one of the major arguments for plate
tectonics.
benthic The portion of the marine environment
inhabited by marine organisms that live in
or on the bottom of the ocean.
benthos Bottom-dwelling marine organisms.
Bergen school Meteorology. A school of
analysis founded in 1918 by the Norwegian
physicist Vihelm Bjerknes (1862–1951), his son
Jacob Bjerknes (1897–1975), Halvor Solberg
(1895– ), and Tor Bergeron (1891– ). V. Bjerknes
began his career as a physicist. In the late
1890s, he turned his attention to the dynamics of
atmosphere and oceans. The “circulation theorems”
he developed during this period provided
a theoretical basis for the basic concepts in the
general circulation. During World War I, Bjerknes,
as founding director of the Geophysical Institute
at Bergen, was successful in convincing
the Norwegian government to install a dense network
of surface stations which provided data for
investigating the surface wind field. These studies
led to the concept of fronts and ultimately
to models of the life cycle of frontal cyclones.
In 1919, J. Bjerknes introduced the concept of
warm, cold, and occluded fronts, and correctly
explained their relationship to extratropical cyclones.
By 1926, in collaboration with Solberg