DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

afternoon cloud (Mars)
are the synoptic scale disturbances that are observed
to form and propagate westward in the
cyclonic shear zone to the south of this jet core.
Occasionally African waves are progenitors of
tropical storms and hurricanes in the western
Atlantic. The average wavelength of observed
African wave disturbance is about 2500 km and
the westward propagation speed is about 8 m/s.
afternoon cloud (Mars) Afternoon clouds
appear at huge volcanos such as Elysium Mons,
Olympus Mons, and Tharsis Montes in spring
to summer of the northern hemisphere. Afternoon
clouds are bright, but their dimension is
small compared to morning and evening clouds.
In their most active period from late spring to
early summer of the northern hemisphere, they
appear around 10h of Martian local time (MLT),
and their normal optical depths reach maximum
in 14h to 15h MLT. Their brightness seen from
Earth increases as they approach the evening
limb. Afternoon clouds show a diurnal variation.
Sometimes afternoon clouds at Olympus
Mons and Tharsis Montes form a W-shaped
cloud together with evening clouds, in which the
afternoon clouds are identified as bright spots.
The altitude of afternoon clouds is higher than
the volcanos on which they appear. See evening
cloud, morning cloud.
aftershocks Essentially all earthquakes are
followed by a sequence of “aftershocks”. In
some cases aftershocks can approach the main
shock in strength. The decay in the number of
aftershocks with time has a power-law dependence;
this is known as Omori’s law.
ageostrophic flow The flow that is not
geostrophic. See geostrophic approximation.
agonic line A line of zero declination. See
declination.
air The mixture of gases near the Earth’s surface,
composed of approximately 78% nitrogen,
21% oxygen, 1% argon, 0.035% carbon dioxide,
variable amounts of water vapor, and traces of
other noble gases, and of hydrogen, methane,
nitrous oxide, ozone, and other compounds.
© 2001 by CRC Press LLC
airfoil probe A sensor to measure oceanic
turbulence in the dissipation range. The probe
is an axi-symmetric airfoil of revolution that
senses cross-stream velocity fluctuations u =
|u | of the free stream velocity vector W (see figure).
Airfoil probes are often mounted on vertically
moving dissipation profilers. The probe’s
output is differentiated by analog electronic circuitstoproducevoltagefluctuationsthatareproportional
to the time rate of change ofu, namely
∂u(z)/∂t, where z is the vertical position. If
the profiler descends steadily, then by the Tayler
transformation this time derivative equals velocity
shear ∂u/∂z = V −1 ∂u(z)/∂t. This microstructure
velocity shear is used to estimate
the dissipation rate of turbulent kinetic energy.
airglow Widely distributed flux predominately
from OH, oxygen, and neon at an altitude
of 85 to 95 km. Airglow has a brightness of
order 14 magnitudes per square arcsec.
air gun An artificial vibration source used
for submarine seismic exploration and sonic
prospecting. The device emits high-pressured
air in the oceanic water under electric control
from an exploratory ship. The compressed air
is conveyed from a compressor on the ship to
a chamber which is dragged from the stern.
A shock produced by expansion and contraction
of the air in the water becomes a seismic
source. The source with its large capacity and
low-frequency signals is appropriate for investigation
of the deeper submarine structure. An air
gun is most widely used as an acoustic source
for multi-channel sonic wave prospecting.
Airy compensation The mass of an elevated
mountain range is “compensated” by a low density
crustal root. See Airy isostasy.
Airy isostasy An idealized mechanism of
isostatic equilibrium proposed by G.B. Airy in
1855, in which the crust consists of vertical rigid
rock columns of identical uniform density ρc
independently floating on a fluid mantle of a
higher density ρm. If the reference crustal thickness
is H , represented by a column of height
H , the extra mass of a “mountain” of height h
must be compensated by a low-density “mountain
root” of length b. The total height of the