DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

operates between 115 and 1100 nm, offers spectral
resolving power ranging from 150 to 100000
and long slit capabilities.
Hubble time Inverse of Hubble constant;
H−1 o = 9.78h−1 × 109 years corresponding to
the time it would take the universe to reach its
present size expanding at constant rateHo. Here
h=H0 (100 km/sec/Mpc). See Hubble radius.
humic substance High molecular weight organic
compounds resulting from plant decay, especially
terrestrial plants; water-soluble soil humic
substance imparts a yellow color to water.
See also colored dissolved organic matter.
hurricane Intense tropical cyclone of the
Atlantic Ocean, Caribbean region, and on the
northeastern coast of Australia. Similar types
of tropical cyclone appearing over the western
Pacific are called typhoons. A hurricane is a
circulatory strong wind system (180 km/h or so)
with low central pressure (below 950 mb), and
is confined to a few hundred kilometers. In the
northern hemisphere the wind rotation is counterclockwise.
Huygens principle In radiation fields, the
phase front at any instant can be thought of as the
locus of radiating sources, and the phase front at
the next instant is obtained by a phase coherent
addition of the radiation from all those sources.
Hyades An open cluster of about 100 stars
visible to the naked eye in the constellation of
Taurus (45 parsecs from the sun, at right ascension
4 h 27 m , declination +16 ◦ 0 ′ ). The Hyades
cluster is about 600 million years old.
hybrid topological defect Different symmetry
breaking schemes in quantum field theory
allow for the possibility of formation of various
kinds of topological defects, like monopoles,
cosmic strings, and domain walls, in the early
universe. Realistic grand unified models generally
involve several phase transitions and therefore
one type may become attached to another,
and the system will end up, say, with
walls bounded by strings or strings connecting
monopoles. Such mixed configurations are
calledhybridtopologicaldefectsandaredynam-
© 2001 by CRC Press LLC
hydraulic depth
ically unstable since no topological constraint
can be applied on them. See cosmic phase transition,
cosmic topological defect, grand unification,
Langacker–Pi mechanism.
hydraulic head (piezometric head) The
equivalent height of a fluid required to maintain
a pressure hydrostatically. The hydraulic
head is of practical meaning only when defined
for a uniform fluid density. For a flow field with
variable fluid density, a reference head is often
defined using a reference density. Given density
ρ, the hydraulic headh is defined as
h= p
ρg +z
where p is fluid pressure, g is gravitational acceleration,
and z is elevation measured from an
arbitrary reference point. In groundwater hydrology,
the hydraulic head is also called the
piezometric head.
hydraulic conductivity (K) The ability of a
porous medium to transmit fluid is dependent on
both the properties of the fluid and the medium:
ρg
K=κ
µ
where κ is the intrinsic permeability of the
porous medium, ρg is the fluid weight, and µ
is the viscosity of the fluid. Hydraulic conductivity
is generally a tensor because of κ.
It has the dimension of velocity. See Darcy’s
law. The hydraulic conductivity of natural substances
ranges over many orders of magnitude,
from 10 −21 ms −1 for unfractured crystalline
rock to 10 −2 ms −1 for karst limestone and
gravel. The hydraulic conductivity decreases
very rapidly as the medium becomes unsaturated,
because the larger pores of the subsurface
become airfilled first, causing flow to occur
in smaller pores which conduct water at much
lower flow rates according to Poiseuille’s law.
We therefore write the hydraulic conductivity in
unsaturated systems as K(θ), indicating that it
is a function of the volumetric moisture content.
hydraulic depth The cross-sectional area of
a free-surface flow (such as a river) divided by
the top width of the flow.
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