Aeronautical Terms beginning with O

Obscuration

Denotes sky hidden by surface-based. obscuring phenomena and vertical visibility restricted overhead.

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Obscuring phenomena

Any hydrometeor or lithometeor other than clouds; may be surface based or aloft.

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Occluded front

A frontal occlusion occurs when a fast-moving cold front catches up with a slow moving warm front. The difference in temperature within each frontal system is a major factor in determining whether a cold or warm front occlusion occurs.

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Occluded Front

The front formed by a cold front overtaking a warm front and lifting the warm air above the Earth’s surface. An occlusion (or frontal occlusion) forms when an air mass is trapped between two colder air masses and is forced to higher and higher levels.

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Occlusion

Same as occluded front—a composite of two fronts as a cold front overtakes a warm front or quasi-stationary front. (commonly called occlusion or frontal occlusion).

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One-Minute Weather

The most recent 1-minute update weather broadcast based on ASOS/AWOS measurements and available to a pilot from an uncontrolled airport ASOS/AWOS.

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Orographic

Of, pertaining to, or caused by mountains as in orographic clouds, orographic lift, or orographic precipitation.

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Orographic Cloud

A cloud that usually results from air flowing upslope from terrain and being cooled adiabatically.

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Outside air temperature (OAT)

The measured or indicated air temperature (IAT) corrected for compression and friction heating. Also referred to as true air temperature.

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Outside Air Temperature (OAT)

The measured or indicated air temperature outside the aircraft that is uncorrected.

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Ozone

An unstable form of oxygen; heaviest concentrations are in the stratosphere; corrosive to some metals; absorbs most ultraviolet solar radiation.

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