DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

particle forces due to electrical charges are significant
compared to particle weight.
clear air turbulence (CAT) Vigorous smallscale
motions within the lowest kilometer of the
atmosphere. Most clear air turbulence is the result
of small-scale wavelike undulations that develop
spontaneously when the vertical shear of
the horizontal flow exceeds some critical value.
cleft, polar See cusp, polar.
climate regimes (Charney and DeVore,
1979). The equilibrium states in the extratropical
circulation in the atmosphere.
climatic optimum Mid-Holocene event in
which summer temperatures in the Northern
hemisphere averaged 1 to 2 ◦ C above current
temperatures. Dated as the period approximately
8000 to 5000 years ago.
closed magnetosphere A model of a magnetosphere
(usually the Earth’s), in which all
planetary magnetic field lines are confined inside
some surface, usually identified with the
magnetopause. The Earth’s magnetosphere resembles
a closed magnetosphere, except for two
details: (1) on the night side, field lines extend
to great distances and no well-defined closure
surface has been observed there; (2) indications
exist that a small amount of interconnection usually
exists. See open magnetosphere.
closed universe A model of the universe that
is finite in total volume and age. It evolves from
a “big bang” to a point of maximum expansion
before contracting back to a “big crunch” of high
density and temperature.
cloudclassification Aschemeofdistinguishing
and grouping clouds based on their appearance,
elevation, and the physical processes generating
them. The World Meteorological Organization
classifies 10 genera in three major
groups — cumulus, stratus, and cirrus, by criteria
essentially according to the cloud formation
processes. The major cloud genera are, cirrus
(Ci), cirrocumulus (Cc), cirrostratus (Cs), altocumulus
(Ac), altostratus (As), nimbostratus
(Ns), stratocumulus (Sc), stratus (St ), cumulus
© 2001 by CRC Press LLC
coastal jet
(Cu), and cumulonimbus (Cb). Clouds can also
be referred to, according to their composition,
as water clouds, ice clouds, or mixed clouds.
clump star A metal-rich star of a few solar
masses that has begun helium burning. Because
of the difference in opacity, such stars do not
reach the giant branch but instead spend a long
period of time in the red clump in the HR diagram,B−V=
1mag, and M= 0mag.
CNO cycle (or tricycle) A set of nuclear
reactions in which hydrogen is fused to helium,
with the liberation of about 7 × 10 18 ergs
per gram, 5 to 10% of which is lost in neutrinos,
depending on the precise temperature
at which the reactions occur, generally in the
range 15–25 ×10 6 K. These reactions, which
require the presence of carbon, nitrogen, or
oxygen as catalysts, are the primary source
of energy for stars larger than about 2 solar
masses during their main sequence lifetimes
and for all stars during the red giant phase.
They are the primary source of nitrogen in the
universe and contribute to the production of
13 C, 15 N, 17 O, 18 O, and, through extensions
toheaviercatalystsathighertemperatures, probably
also to 19 F, 21 Ne, 22 Ne, 23 Na, and perhaps
26 Al.
The primary reaction sequence is (see nuclear
reactions for explanation of the symbols):
12 C(p,γ) 13 N(e + νe) 13 C(p,γ) 14 N(p,γ)
15 O(e + νe) 15 N(p,α) 12 C,
thus the net effect is to convert four protons (hydrogen
atoms) to one helium nucleus or alpha
particle, returning the 12 C to its original form.
Other closures include
15 16 17 +
N(p,γ) O(p,γ) F(e νe) 17 O(p,γ)
18 +
F(e νe) 18 O(p,α) 15 N.
See nuclear reactions.
cnoidal wave A nonlinear theory for periodic
waves, attributed to Korteweg and Devries
(1895), in which the water surface profile is expressed
in terms of a Jacobian elliptical integral,
cn(u).
coastal jet a coastal current system that intensifies
toward the coast.