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Words begining with "O"
A spot in a desert made fertile by water, which normally
originates as groundwater.
Occluded Front (Occlusion)
A composite of two fronts formed
as a cold front overtakes a warm front. A cold occlusion results when
the coldest air is behind the cold front. The cold front undercuts
the warm front and, at the Earth's surface, coldest air replaces
A warm occlusion occurs when the coldest air lies ahead of the
warm front. Because the cold front can not lift the colder air mass,
it rides piggyback up on the warm front over the coldest air
The salt water surrounding the great land masses. The land masses
divide the ocean into several distinct portions, each of which also
is called an ocean.
The oceans include:
- Pacific Ocean
- Atlantic Ocean
- Indian Ocean
- Arctic Ocean
The unit of electrical resistance, equal to the resistance of a
circuit in which an electromotive
force of one volt maintains a
current of one ampere. Named for
German physicist Georg S. Ohm (1787-1854).
The path described by a heavenly body in its periodic revolution.
Earth satellite orbits with inclinations near 0 degrees are called
equatorial orbits because the satellite stays nearly over the
equator. Orbits with inclinations near 90 degrees are called polar
orbits because the satellite crosses over (or nearly over) the north
and south poles. See inclination.
See period decay.
An imaginary gigantic flat plate containing an Earth satellite's
orbit. The orbital plane passes through the center of the Earth.
An almost colorless, gaseous form of oxygen with an odor similar
to weak chlorine. A relatively unstable compound of three atoms of
oxygen, ozone constitutes - on the average - less than one part per
million (ppm) of the gases in the atmosphere (peak ozone
concentration in the stratosphere can get as high as 10 ppm). Yet
ozone in the stratosphere absorbs nearly all of the biologically
damaging solar ultraviolet radiation before it reaches the Earth's
surface where it can cause skin cancer, cataracts, and immune
deficiencies, and can harm crops and aquatic ecosystems. See
The Ozone Cycle
- Destruction of ozone is a catalytic process
- Chlorofluorocarbon atoms in the stratosphere are split by
ultraviolet radiation and release their chlorine atom.
- The chlorine atom takes one oxygen atom from the unstable
ozone molecule and forms chlorine monoxide leaving an ordinary
- When a free atom of oxygen collides with the chlorine monoxide
the two oxygen atoms form a molecule releasing the chlorine atom
to destroy more ozone
Ozone is produced naturally in the middle and upper stratosphere
through dissociation of molecular oxygen by sunlight. In the absence
of chemical species produced by human activity, a number of competing
chemical reactions among naturally-occurring species - primarily
atomic oxygen, molecular oxygen, and oxides of hydrogen and nitrogen
- maintains the proper ozone balance.
In the present-day stratosphere, this natural balance has been
altered, particularly by the introduction of man-made chlorofluorocarbons.
If the ozone decreases, the ultraviolet radiation at the Earths
surface will increase. See qreenhouse
Tropospheric ozone is a by-product of the photochemical
(light-induced) processes associated with air pollution. See
Ozone in the troposphere can damage plants and humans.
A large area of intense stratospheric ozone depletion over the
Antarctic continent that typically occurs annually between late
August and early October, and generally ends in mid-November. This
severe ozone thinning has increased conspicuously since the late
seventies and early eighties. This phenomenon is the result of
chemical mechanisms initiated by man-made chlorofluorocarbons (see
CFCs). Continued buildup of CFCs is
expected to lead to additional ozone loss worldwide.
The thinning is focused in the Antarctic because of particular
meteorological conditions there. During Austral spring (September and
October in the Southern Hemisphere) a belt of stratospheric winds
encircles Antarctica essentially isolating the cold stratospheric air
there from the warmer air of the middle latitudes. The frigid air
permits the formation of ice clouds that facilitate chemical
interactions among nitrogen, hydrogen, and chlorine (elevated from
CFCs) atoms, the end product of which is the destruction of ozone.
The layer of ozone that begins approximately 15 km above Earth and
thins to an almost negligible amount at about 50 km, shields the
Earth from harmful ultraviolet radiation from the sun. The highest
natural concentration of ozone (approximately 10 parts per million by
volume) occurs in the stratosphere at approximately 25 km above
Earth. The stratospheric ozone concentration changes throughout the
year as stratospheric circulation changes with the seasons. Natural
events such as volcanoes and solar flares can produce changes in
ozone concentration, but man-made changes are of the qreatest
Ozone Measuring Satellite Instruments
Satellite-based ozone-measuring instruments can measure ozone by
looking at the amount of ultraviolet absorption reflected from the
Earth's surface and clouds. Some instruments provide data within the
different levels of the atmosphere. The Total Ozone Mapping
Spectrometer (TOMS) maps the total
amount of ozone between ground and the top of the atmosphere.
The amount and distribution of ozone molecules in the stratosphere
varies greatly over the globe, changing in response to natural cycles
such as seasons, sun cycles, and winds. Utilizing satellites has
enabled scientists to assess ozone levels simultaneously over the
entire Earth, and has led them to conclude that global ozone levels
are being depleted.
Rapid, transient, polar-ozone depletion. These depletions, which
take place over a 50-kilometer squared area, are caused by weather
patterns in the upper troposphere. The decrease in ozone during a
mini-hole event is caused by transport, with no chemical depletion of
ozone. However, the cold stratospheric temperatures associated with
weather systems can cause clouds to form that can lead to the
conversion of chlorine compound from inert to reactive forms. These
chlorine compounds can then produce longer-term ozone reductions
after the mini-hole has Passed
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