OBSCURATION
Any phenomena in the atmosphere, excluding
precipitation, that reduces horizontal visibility. According to the National
Weather Service, some of the obstructions to visibility include
blowing and widespread dust, fog (including freezing fog and patchy
fog), haze, mist, sand and blowing sand, smoke, blowing spray, and volcanic
ash. It is reported as "X" in an observation and on the METAR.
Related term: partial obscuration
OBSERVATION
In meteorology, the evaluation of one or more meteorological elements, such as temperature,
pressure, or wind, that describe the state of the atmosphere,
either at the earth's surface or aloft. An observer is one who records
the evaluations of the meteorological elements.
OCCLUDED FRONT
Also known as an occlusion, it is a complex front
formed when a cold front overtakes a warm front. It develops when three
thermally different air masses conflict. The type of frontal boundary
they create depends on the manner in which they meet. Related terms:
cold front and warm
front
OCEAN
The intercommunicating body of salt water occupying
the depressions of the earth's surface, or one of its major primary
subdivisions, bounded by the continents, or the equator,
and other imaginary lines. A sea is subdivision of an ocean.
OCEANOGRAPHY
The study of the ocean, embracing and integrating
all knowledge pertaining to the ocean's physical boundaries, the chemistry
and physics of sea water, and
marine biology.
OMEGA BLOCK
A warm high aloft which has become
displaced and is on the polarward side of the jet
stream. It frequently occurs in the late winter and early spring
in the Northern Hemisphere. The name comes from its resemblance to the
Greek letter, Omega, when analyzed on upper air charts. Related term:
blocking high
OPAQUE
A condition where a material, such as a cloud, blocks the passage of radiant energy,
especially light. Opaque sky cover refers to the amount of sky cover
that completely hides all that might be above it.
OROGRAPHIC LIFTING
Where the flow of air is forced up and over barriers such as highlands
or mountains. Moist air being forced aloft begins to cool, consequently
condensation forms, and rain or snow begins to fall. By the time the air reaches the leeward
side of the barrier, it sinks and warms, resulting in decreasing relative
humidity, cessation of precipitation,
and the dissipation of clouds. May be called an orographic uplift.
OUTFLOW
Also referred to as an outflow boundary, it is the outward flow of air from a system,
such as a thunderstorm. It is the result of cold downdrafts
and its passage includes a wind shift and temperature drop. Related
terms: bubble high and meso
high
OVERCAST
The amount of sky cover for a cloud layer that is 8/8ths, based on the summation layer
amount for that layer.
OVERRUNNING
This occurs when a relatively warm air mass is
forced above a cooler air mass of greater density. Weather generally associated with
this event includes cloudiness, cool temperatures, and steady precipitation.
OXYGEN(O2)
A colorless, tasteless, odorless gas that is the second most abundant constituent of dry air,
comprising 20.946%.
OZONE (O3)
A nearly colorless
gas and a form of oxygen (O2). It is composed of an oxygen molecule
made up of three oxygen atoms instead of two.
OZONE LAYER
An atmospheric layer that contains a high proportion of oxygen that exists as ozone.
It acts as a filtering mechanism against incoming ultraviolet
radiation. It is located between the troposphere and the stratosphere,
around 9.5 to 12.5 miles (15 to 20 kilometers) above the earth's surface.
