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Words begining with "V"
Van Allen Belt or Van Allen Radiation Belt
Doughnut-shaped regions encircling Earth and containing high
energy electrons and ions trapped in the Earths magnetic field (the
magnetic field has definite boundaries, and is distorted into a
tear-drop shape by the solar
wind). Explorer 1, launched by NASA in 1958, discovered this
intense radiation zone. These regions are called the inner and outer
Van Allen radiation belts, named after the scientist who first
observed them. See magnetosphere.
Vernal Equinox
The beginning of spring in the Northern Hemisphere. The time/day
that the sun crosses the equatorial plane going from south to north.
Very High Frequency (VHF)
Referring to the 50-400 MHz portion of the radio frequency
spectrum. Polar-orbiting satellite transmissions (APT)
are made in the 136-138 MHz range using FM
modulation.
Video
A signal containing information on the brightness levels of
different portions of an image along with information on line and
frame synchronization. In the case of satellite signals, the video
information is transmitted in the form of an AM
modulated subcarrier.
Visible
That part of the electromagnetic
spectrum to which the human eye is sensitive, between about 0.4
and 0.7 micrometers. See spectrum.
Visible Infrared Spin Scan Radiometer (VISSR)
High-resolution, multi-spectral imaging system flown on the
pre-GOES-8 geostationary GOES
spacecraft. Similar systems are flown on the METEOSAT
and GMS spacecraft.
Volcano
A naturally occurring vent or fissure at the Earth's surface
through which erupt molten, solid, and gaseous materials. Volcanic
eruptions inject large quantities of dust, gas, and aerosols into the
atmosphere. A major component of volcanic clouds is sulfur dioxide, a
strong absorber of ultraviolet radiation. Chemical interactions
between sulfur dioxide and water cause sulfuric acid aerosols which
can scatter some of the incident solar radiation back to space, thus
causing a global cooling effect. For example, Mt. Pinatubo in the
Philippines erupted in June 1991, and in the following year the
global surface temperature was observed to decrease by about O.3
degrees C.
Volt
The unit of electromotive
force, or difference of potential, which will cause a current of
one ampere to flow through a
resistance of one ohm. Named for
Italian physicist Alessandro Volta (1745- 1827).
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