oblique photography. A type of aerial photography in which photographs are taken with the camera pointed at an angle, rather than straight down, as is done in aerial mapping.
oblique shock wave (aerodynamics). A shock wave that forms on a sharp-pointed object moving through the air at a speed greater than the speed of sound. Air passing through an oblique shock wave is slowed down, but not to a subsonic speed, and its static pressure is increased. See illustration for shock wave.
oblique triangle. A closed three-sided, plane (flat) figure that does not contain any right angles (90° angles).
oblong shape. A shape that is neither square nor round, but can be either an elongated (stretched) square or circle. An oblong circle is called an ellipse, and an oblong square is called a rectangle.
obscuration (meteorology). A term meaning that the sky is hidden by surface-based obscuring phenomena
and vertical visibility is restricted overhead.
obscuring phenomena (meteorology). Any dry particles or particles of liquid water other than clouds.
Obscuring phenomena may be either surface-based or aloft.
observation aircraft. Aircraft used by the military to fly behind enemy lines and observe the movement of
troops or the effects of artillery fire.
obsolete. Out of date or no longer in use because of being replaced with something newer.
obstacle (air traffic control). An existing object, object of natural growth, or terrain at a fixed
geographical location or which may be expected at a fixed location within a prescribed area, with reference
to which vertical clearance is or must be provided during flight operation.
obstacle free zone. See OFZ.
obstruction light (airport lighting). A light or one of a group of lights, usually red or white, frequently
mounted on a surface structure or natural terrain to warn pilots of the presence of an obstruction.
obtuse angle. An angle of more than 90°, yet less than 180°. An obtuse angle may be called an open angle.
obtuse triangle. A triangle that contains an obtuse angle (an angle greater than 90°). See illustration for oblique triangle.
occluded front (meteorology). A composite of two fronts. An occluded front forms when a cold front overtakes a warm front or a quasi-stationary front.
oceanic airspace (air traffic control). Airspace over the oceans of the world, considered international airspace, where oceanic separation and procedures per ICAO are applied. Responsibility for the provisions of air traffic control service in this airspace is delegated to various countries, based generally upon geographic proximity and the availability of the required resources.
oceanic published route (air traffic control). A route established in international airspace and charted or described in flight information publications such as Route Charts, DOD En Route Charts, Chart Supplements, NOTAMs, and Track Messages.
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