18WuD2x

44 atmospheric bending • atomic radiation
atmospheric bending The refraction or reflection
of electromagnetic waves by the troposphere or
ionosphere. See ATMOSPHERIC REFLECTION.
atmospheric duct A tropospheric stratum, often
associated with temperature inversions, lake effects,
or weather fronts, through which electromagnetic
energy at ultra-high and microwave
frequencies is efficiently propagated for long distances.
atmospheric electricity Static electricity present
in the atmosphere, which evidences itself in disturbance
of radio communications and in displays
of lightning.
atmospheric noise Receiver noise resulting from
ATMOSPHERIC ELECTRICITY. Also called sferics
or static.
atmospheric pressure Abbreviation, atm press. 1.
The pressure exerted by the earth’s atmosphere,
as indicated by a barometer at sea level; normally
between 29 and 31 inches of mercury. 2. A pressure
of 1.013 × 10 6 dynes per square centimeter.
See ATMOSPHERE, 2.
atmospheric radio wave See SKYWAVE.
atmospheric radio window The band of frequencies
(approximately 10 MHz to 10 GHz), including
radio waves that can penetrate the earth’s troposphere
and ionosphere.
atmospheric reflection The return of a radio wave
to earth, resulting from reflection by an ionized
portion of the atmosphere.
atmospheric refraction 1. Downward bending of
radio waves as a result of variations in the dielectric
constant of the troposphere. 2. Downward
bending of radio waves in the ionosphere, resulting
in long-range propagation at high frequencies.
atmospheric scatter 1. The scattering of very-high
frequency ( VHF) and ultra-high frequency (UHF)
radio waves by the lower atmosphere. 2. Communication
via scattering of VHF and UHF radio
waves in the lower atmosphere.
atmospherics See ATMOSPHERIC NOISE.
atom 1. The smallest material particle that displays
the unique characteristics and properties of
an element. Atoms consist of a dense, positively
charged central nucleus, around which less-massive,
negatively charged electrons “swarm” at definite
levels called shells. Also see BOHR ATOM
and RUTHERFORD ATOM. 2. In a computercompiling
operation, an operator or operand.
atomechanics The physics of electron movement.
atomic battery A battery in which atomic energy is
converted into electrical energy.
atomic charge The electrification (i.e., the electron
charge) exhibited by an ion.
atomic clock Also called atomic time standard. A
highly accurate electronic clock, driven by the
characteristic oscillations of certain atoms.
atomic energy Energy released by the FUSION or
FISSION of atomic nuclei. Also see ATOMIC
POWER.
atomic fission See FISSION.
atomic frequency The natural vibration frequency
of an atom.
atomic fusion See FUSION.
atomic pile See REACTOR, 2.
atomic mass unit Abbreviated amu. A unit that
expresses the relative mass of an elemental isotope.
One amu is equal to 1⁄12 of the atomic mass
of carbon 12 (C12). A neutron has a mass of
roughly one amu.
atomic migration The transfer or “wandering” of a
valence electron between or among atoms in a
single molecule.
atomic number The number of protons in the nucleus
of an atom. Also, the number of electrons if
the atom is electrically neutral. For example, the
atomic number for copper is 29, indicating 29
protons in the nucleus. An electrically neutral
atom of copper has 29 electrons. The atomic
number uniquely identifies an element.
atomic radiation The emission of radiant energy
by radioactive substances.