The temperature gradient defining the → radiative equilibrium
condition in a region. It is expressed as:
dT/dr = (1 - 1/ γ)((T / P)(dP / dr),
where T and P are temperature and pressure, dT / dr and
dP / dr temperature and
pressure gradients respectively, and γ = C_{P} / C_{V}.
For radiative equilibrium to be stable against → convection,
the actual temperature gradient must be less than the adiabatic temperature gradient, i.e.
|dT /dr|_{rad} < |dT /dr|_{ad}.
See also → Schwarzschild's criterion.

1) General: Degree of slope.
2) Physics: Change in the value of a quantity (as temperature, pressure) with
change in a given variable.
3) Math.: A differential → operator
(symbol → nabla,
∇) that, operating upon a function (f) of several variables, creates a
→ vector
whose coordinates are the → partial derivatives of
the function: ∇f = (∂f/∂x)i
+ (∂f/∂y)j + (∂f/∂z)k.
The gradient of a → scalar function is a vector function.

From L. gradient-, gradiens, pr.p. of gradi "to walk, go,"
from grad- "walk" + -i- thematic vowel + -ent suffix of conjugation.

Ziné "ladder, steps, stair," may be related to ciné, from
cidan "to place (something) above/upon (another similar thing);"
cf. Lori râ-zina, Yazdi râ-cina "stairs," Nâyini orcen "stairs,
ladder;" the phoneme change -c- into -z-, as in
gozidan, gozin-/cidan, cin- both deriving from Proto-Ir. *cai-
"to heap up, gather, collect."

metallicity gradient

زینهی ِ فلزیگی

zine-ye felezigi

Fr.: gradient de métallicité

The decrease in the → abundances of
→ heavy elements in a → disk galaxy as
a function of distance from the center. Radial metallicity gradients are observed in
many galaxies, including the → Milky Way and other galaxies of the
→ Local Group. In the case of the Milky Way,
several objects can be used to determine the gradients: → H II regions,
→ B stars, → Cepheids,
→ open clusters, and → planetary nebulae.
The main diagnostic elements are oxygen, sulphur, neon, and argon in photoionized nebulae,
and iron and other elements in Cepheids, open clusters, and stars.
Cepheids are probably the most accurate indicators of abundance gradients in the
Milky Way. They are bright enough to be observed at large distances, so that accurate distances
and spectroscopic abundances of several elements can be obtained.
Average abundance gradients are generally between -0.03 → dex/kpc
and -0.10 dex/kpc, with a a flattening out of the gradients at large galactocentric
distances (≥ 10 kpc). The existence of these gradients offers the opportunity to test
models of → chemical evolution of galaxies and stellar
→ nucleosynthesis.

At a point, the rate of change of potential V, with distance x, measured
in the direction in which the variation is a maximum. The intensity F of the
field is proportional to the potential gradient, but is oppositely directed:
F = -dV/dx.

The pressure difference between two adjacent regions of a fluid that results in
a force being exerted from the high pressure region toward the low pressure region.

A condition in which there is an excess of the actual temperature gradient
over the → adiabatic temperature gradient
corresponding to the same pressure gradient.
A region with superadiabatic temperature gradient is convectively unstable.
→ Hayashi forbidden zone.

A physical quantity that describes the rate of change of temperature
with displacement in a given direction from a given reference point.
Same as → thermal gradient.

A vector quantity that depends on the distribution of temperature in three dimensions
with respect to a given point. The magnitude and orientation of the maximum thermal
gradient are given by:
∇T = (∂T/∂x)i + (∂T/∂y)j
+ (∂T/∂z)k,
where T is the temperature distribution function in three dimensions, and
i, j, and k are the unit vectors along the
x, y, and z axes defining the temperature field.
Same as → temperature gradient.

Fluid Mechanics:
The rate at which the velocity changes with the distance across the flow.
When a fluid flows past a stationary wall, the
fluid right close to the wall does not move. However, away from the
wall the flow speed is not zero. Therefore a velocity gradient exists, which is
due to adhesive, cohesive, and frictional forces. The
amount of the velocity gradient is characteristic of the fluid.