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Observational network- A group of meteorological observing stations spread over a given area for a specific purpose.

Ocean station vessel- (Also called ocean weather ship.) An oceangoing vessel assigned to patrol an ocean station. These ships are specially equipped to take comprehensive meteorological observations of conditions both at the surface and aloft. The U.S. vessels are provided by the U.S. Coast Guard, and the meteorological personnel and equipment are provided by the National Weather Service.

Ocean weather station- As defined by the World Meteorological Organization, a specific maritime location occupied by a ship equipped and staffed to observe weather and sea conditions and report the observations by international exchange.

Off-center PPI scope- An obsolete type of radar display in which the center of a PPI display is offset from the geometrical center of the oscilloscope.

Ombrometer- (Also called micropluviometer or trace recorder.) Specifically, a rain gauge capable of measuring very small amounts of precipitation. 2. Any rain gauge.

Ombroscope- An instrument that indicates the presence of precipitation. The ombroscope consists of a heated, water-sensitive surface that indicates by mechanical or electrical techniques the occurrence of precipitation.

One-dimensional cloud probe- (Also called optical imaging probe.) An optical particle probe that records the size and shape of the shadow of each cloud particle that intercepts and attenuates the illumination by a laser beam.

Operational Linescan System- The visible and infrared imager carried aboard the DMSP satellites, which provides a variety of moderate (2.7 km at nadir) and high-resolution (0.6 km at nadir) imagery.

Optical counter- Device for counting the concentration of particles in air that takes advantage of the light scattering caused by particles.

Optical hygrometer- A group of hygrometers based on the spectral attenuation of radiation by specific water vapor absorption lines or bands.

Optical imaging probe- (Also called optical array probe.) An optical particle probe that records the size and shape of the shadow of each particle that intercepts and attenuates the illumination by a laser beam.

Optical particle probe- (Also called electro-optical particle probe.) Any of a class of instruments for sizing and counting large numbers of individual aerosols or hydrometeors to characterize populations, or also for imaging and classifying the shapes of individual hydrometeors by measuring the optical illumination, scattering, or attenuation of a laser beam by each particle and recording the result electronically.

Optical rain gauge- A gauge that measures the scintillation in an optical beam produced by raindrops falling between a light source and an optical receiver. By measuring the energy in selected frequency bands of the scintillation spectrum and comparing their ratios, the occurrence of precipitation,type of precipitation, and intensity of precipitation can be estimated.

Optical scattering probe- A type of optical particle probe that measures the forward scattering caused by each particle or ensemble of particles intercepting a laser beam.

Optical Transient Detector- An instrument, designed to detect lightning from space, that was launched on 3 April 1995 aboard the MicroLab-1 satellite into a near polar orbit at an inclination of 70°.

Orthogonal antennas- In radar, a pair of transmitting and receiving antennas, or a single antenna used for transmission and reception, designed for measuring the copolarized signal and the cross- polarized signal.

Oscillator- The general term for an electrical device that generates alternating currents or voltages.

Oscilloscope- (Also called cathode-ray oscilloscope.) An instrument that displays visually on the face of a cathode-ray tube instantaneous voltages of electrical signals. Either the intensity or the displacement of the trace may be controlled by the signal voltage.

Osmometer- A device for measuring soil water amounts, especially under roads; seldom used.

Owens dust recorder- An instrument for rapidly obtaining samples of airborne dust; a type of dust counter.

Ozonesonde- A radiosonde equipped with an instrument to measure the atmospheric concentration of ozone (O3).

Ozone spectrophotometer- (Also called Dobson spectrometer.) A photoelectric spectrophotometer that is used to determine the ozone content of the atmosphere.