DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

potential
Potassium-argon ages are reliable to hundreds
of millions of years.
potential In nonrelativistic conservative mechanical
systems, a scalar function φ from which
the mechanical forces are obtained as proportional
to ∇φ. Examples are the electric potential
and the gravitational potential.
In fluid mechanics, in situations where the
fluid velocity vector v is irrotational, v may be
obtained as the gradient of a velocity potential.
potential density The density that a parcel
of fixed composition would acquire if moved
adiabatically to a given pressure level (called
reference pressure).
potential energy The ability to do a specific
amount of work. In conservative mechanical
systems, work (energy) can be done on the
system and stored there. This stored work is
called potential energy (units of Joules or ergs
in metric systems). A prototypical example is
a system consisting of a stone moved to the top
of a mountain. Work is required to move it to
the top; once there, the system contains potential
energy, which can be recovered (as heat or
as thermal energy) by allowing the stone to roll
down the mountain. A related gravitational example
is in the storage of water behind a dam;
potential energy is converted to work by allowing
the water to flow through a turbine. Electrostatics
provides another example, in which work
must be done to bring a positive charge from infinity
to add to a collection of positive charge.
Such a collection has potential energy which can
be recovered by allowing the charges to freely
accelerate away from one another; the energy
can then be collected as kinetic energy, as the
charges move off to infinity.
Potential energy may also be mechanical (a
compressed spring) or chemical, such as the energy
stored in unstable compounds (e.g., nitroglycerine)
or in flammable substances (e.g., H2
and O2 gases mixed). It also exists in extractable
form in the nuclei of heavy atoms (e.g., U235 ),
which release it as kinetic energy when undergoing
fission; and in a system of neutrons and
protons, which will, at appropriate densities and
pressures, release some potential energy as ki-
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netic, in fusion to form helium and heavier nuclei.
potential height Dynamic height.
potential instability Also called “convective
instability”. Stratification instability caused
by convective activities, i.e., the lower layer
has higher moisture and becomes saturated first
when being lifted, and hence cools thereafter
at a slower rate than does the upper, drier portion,
until the lapse rate of the whole layer
becomes equal to the saturation adiabatic and
any further lifting results in instability. In
general, use ∂θsw
∂θse
< 0or < 0 as the
∂Z ∂Z
criterion of convective instability. (θsw and
θse are pseudo-wet-bulb potential temperature
and pseudo-equivalent potential temperature,
respectively.)
potential temperature The temperature
that a parcel of fixed composition would acquire
if moved adiabatically to a given pressure
level (called reference pressure). For instance
in saltwater, is the temperature that
would be measured after a water parcel, which
shows the measured in situ temperature T, has
been moved isentropically (without exchanging
heat or salt) through the ambient water masses
to a reference depth, usually taken at the surface:
dθ
dz = dT
dz −
dT
dz . The difference between
ad
and T is given by θ(z) = T(z)−
z0 dT
z dz
ad dz′ .
potential theory In geophysics, the area
that utilizes gravitational potentials to determine
the interior structure of a body. Study of a
body’s gravitational potential (obtained by perturbations
on an orbiting body) provides information
about the distribution of interior mass,
how much the primary body varies from hydrostatic
equilibrium, the response of a body to tidal
forces, and how much isostatic equilibrium surface
features have undergone.
potential vorticity (Rossby, 1940) In a shallow
homogeneous layer, the potential vorticity
is a conserved dynamic quantity Q as the ratio
between the absolute vorticity and the thickness
of the layer, defined by Q = (f + ζ)/H, where
f is the planetary vorticity (i.e., Coriolis param-