Obscuration-
In U.S. weather observing practice, the designation for the sky cover when the sky is completely hidden by surface based obscuring phenomena.
Occluded front-
A front that forms as a cyclone moves deeper into colder air.
Ocean conveyor belt-
The global recirculation of water masses that determines todays climate.
Ocean mixing-
Any process or series of processes by which parcels of ocean water with different properties are brought into intimate small scale contact, so that molecular diffusion erases the differences between them.
Offshore wind-
Wind blowing from land to sea. During synoptic conditions of light winds, offshore winds near the surface often occur at night as a component of the land breeze.
Onshore wind-
A wind blowing from water onto land; the wind may be a result of heating differences between land and water or related to synoptic weather patterns.
Open channel flow-
Flow of a fluid with its surface exposed to the atmosphere.
Open system-
A thermodynamic system so chosen that there may be transfer of mass across the boundaries; for example, an air parcel undergoing a pseudoadiabatic expansion.
Optical depth-
The optical thickness measured vertically above some given altitude.Optical depth is dimensionless and may be used to specify many different radiative characteristics of the atmosphere.
Optical mass-
The vertical integral of the density of absorbers between two altitudes; used mainly in determining the transmission through an absorbing gas.
Orbital motion-
In hydrodynamics, the motion of a fluid particle induced by the passage of a progressive gravity wave.
Orographic cloud-
Mountain clouds produced by orographic lifting of moist air to saturation.
Orographic fog-
Fog formed as moist air blows up a mountain slope and becomes saturated.
Orographic lifting-
Ascending air flow caused by mountains.Mechanisms that produce the lifting fall into two broad categories: 1) the upward deflection of horizontal larger scale flow by the orography acting as an obstacle or barrier; or 2) the daytime heating of mountain surfaces to produce anabatic flow along the slopes and updrafts in the vicinity of the peaks.
Outer layer-
For flow over a hill, the top layer in the boundary layer that accelerates relative to its upstream value due to the Bernoulli effect.
Outflow boundary-
A surface boundary formed by the horizontal spreading of thunderstorm cooled air.
Outflow jet-
Nocturnal cold air jet flowing out of the mouth of a valley or canyon as it opens onto a plain.
Overshooting top-
A domelike protrusion above a cumulonimbus anvil, representing the intrusion of an updraft through its equilibrium level.
Oxidation-
Reaction of a substance with oxygen or incorporation of oxygen into a molecule.
Ozone-
A nearly colorless gas, it is a very strong absorber of ultraviolet radiation, and the presence of the ozone layer in the upper atmosphere provides an ozone shield that prevents dangerous radiation from reaching the earths surface.
