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Occluded mesocyclone

A mesocyclone in which air from the rear-flank downdraft has completely enveloped the circulation at low levels, cutting off the inflow of warm unstable low-level air.

Occlusion

A frontal system which forms when a cold front overtakes a warm front. When the air behind the front is cooler than the air ahead, it is called a cold occlusion, when the air ahead is milder, it is a warm occlusion.

Offshore

The coastal water zone seaward of inshore waters.

Orographic

Related to, or caused by, physical geography (such as mountains or sloping terrain).

Orographic lift

Lifting of air caused by its passage up and over mountains or other sloping terrain.

Orographic rain

Precipitation that results from Orographic lift

Orphan anvil

[Slang] An anvil from a dissipated thunderstorm, below which no other clouds remain.

Outflow boundary

A storm-scale or mesoscale boundary separating thunderstorm-cooled air (outflow) from the surrounding air; similar in effect to a cold front, with passage marked by a wind shift and usually a drop in temperature. Outflow boundaries may persist for 24 hours or more after the thunderstorms that generated them dissipate, and may travel hundreds of kilometres from their area of origin. New thunderstorms often develop along outflow boundaries, especially near the point of intersection with another boundary (cold front, dry line, another outflow boundary, etc.; see triple point).

Overcast

Describes cloud cover when 8/8ths of the sky is covered in cloud.

Overhang

Radar term indicating a region of high reflectivity at middle and upper levels above an area of weak reflectivity at low levels. (The latter area is known as a weak-echo region, or WER.) The overhang is found on the inflow side of a thunderstorm (normally the south or southeast side). See BWER.

Overrunning

A weather pattern in which a relatively warm air mass is in motion above another air mass of greater density at the surface. Embedded thunderstorms sometimes develop in such a pattern; severe thunderstorms (mainly with large hail) can occur, but tornadoes are unlikely.

Overrunning often is applied to the case of warm air riding up over a retreating layer of colder air, as along the sloping surface of a warm front. Such use of the term technically is incorrect, but in general it refers to a pattern characterized by widespread clouds and steady precipitation on the cool side of a front or other boundary.

Overshooting top

(or penetrating top) A dome-like protrusion above a thunderstorm anvil, representing a very strong updraft and hence a higher potential for severe weather with that storm. A persistent and/or large overshooting top (anvil dome) often is present on a supercell. A short-lived overshooting top, or one that forms and dissipates in cycles, may indicate the presence of a pulse storm or a cyclic storm. See HP storm, LP storm, and supercell.

Oxygen

The second most abundant gas in air, comprising 21% by volume. It is a colourless and odourless gas.

Ozone

One of several gases that make up the Earth's atmosphere. It is the triatomic form of oxygen and makes up approximately one part in three million of all of the gases in the atmosphere. If all the ozone contained in the atmosphere from the ground level up to a height of 60 km could be assembled at the earth's surface, it would comprise a layer of gas only about 3 millimetres thick. Ozone is toxic at high concentrations because it reacts strongly with other molecules.

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