DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

Tethys system. Calypso orbits at the other. Its
orbit has a semimajor axis of 2.95 ×10 5 km. Its
size is 17 × 14 × 13 km, but its mass has not
been measured. It has a geometric albedo of 0.5
and orbits Saturn once every 1.888 Earth days.
temperature (1.) In thermodynamics, the
integrating factor in the first law of thermodynamics:
dE=dU+TdS
where the energy E, the internal energy U, and
the entropy S are functions of the state of the
equilibrium system, and dE, dU,dS are their
functional differentials; a function proportional
to the pressure of a hypothetical perfect gas held
at constant volume.
(2.) In statistical mechanics, a measure of the
translational molecular kinetic energy.
(3.) The degree of hotness measured on a
conventional temperature scale.
(Definitions (1.) and (2.) agree for equilibrium
systems, as does (3.) for some appropriate
range of temperature specific to the device.)
(4.) In equilibrium photon dynamics, the
inverse multiplier of E/k (where k = Boltzmann’s
constant) in the Plack distribution f =
f(E/kT); the temperature of a black body radiation.
In non-equilibrium situations, e.g., astronomical
observations, a derived measure which
agrees with one of the above definitions for some
range of applicability.
If the object radiates like a true black body, all
of these temperatures are equivalent; however,
a perfect black body in nature is rare. A perfect
black body is thermalized, that is, the atoms or
molecules of the object are in perfect thermodynamic
equilibrium. Most stars have a radiation
distribution with wavelength that closely
but not exactly approximates that of a black
body. The sun, for example, has a radiation temperature
that approximates a black body with
TB = 6300 K; however, its effective temperature,
which compares the total output power to
that of a black body, is better approximated by
aTeff = 5800 K, and the color temperature of
the sun which compares the energy output ratio
over two wavelength intervals is Tc = 6500 K.
See excitation temperature, effective temperature,
blackbody temperature, brightness temperature,
color temperature.
© 2001 by CRC Press LLC
tension (cosmic string)
temperature inversion An increase of atmospheric
temperature with altitude. Under this
condition the typical lapse rate is reversed, and
great stability is created, which strongly damps
vertical motions and vertical turbulent transport.
Wind shear will exist between the top and the
bottom of the temperature inversion layer. A
temperature inversion acts as a ceiling, preventing
further upward convection, and is generally
the limit for cloud development. Marked and
persistent inversions occur at lower levels, with
subsiding air in major anti-cyclonic cells, such
as the Azores high-pressure zone and cold anticyclones
over continents. Temperature inversion
arises for different reasons, such as frontal
inversion, subsidence inversion, trade-wind inversion,
radiation inversion, advective inversion,
turbulence inversion, and stratospheric inversion.
See density inversion.
temperature variance dissipation rate
Measure of the rate at which gradient fluctuations
of temperature are smoothed out by turbulence.
It is defined by
χT = 2κT (∇T ) 2
where κT is the thermal diffusivity, T is the temperature,
z is the vertical coordinate, and ∇ denotes
the gradient operator. The temperature
variance dissipation rate can be regarded as the
thermal equivalent of the dissipation rate of velocity
fluctuations ɛ.
In oceanic or atmospheric studies, generally
only one component of the temperature gradient
is measured, and χT is estimated from
χT 2λκT
2 ∂T
.
∂z
The factor λ is a scaling parameter that reflects
the level isotropy. For a completely isotropic
turbulence field λ = 3.
tension (cosmic string) For a stringlike topological
defect, the quantity
T =−2π rdrT zz ,
where r is the radial coordinate in a cylindrical
reference frame aligned with the string, and T µν
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