O Obscuring phenomenon An atmospheric phenomenon, other than clouds, which obscures a portion of the sky from the point of observation. Also called obscuration.     Observational error The difference between the true value of some quantity and its observed value. Every observation is subject to certain errors.  Systematic errors: affect the whole of a series of observations in nearly the same way. For example, the scale of an instrument may be out of adjustment. These instrument errors can be detected and corrected by comparison with a standard. The personal equation of an observer may lead him or her to make small systematic errors in his or her readings, for example, if the scale is not at eye level.  Random errors: which appear in any series of observations, are generally small and as likely to be positive as negative. Their magnitudes are usually distributed according to the error distribution. Mistakes are widely discrepant readings.     Ombrometer A rain gauge capable of measuring very small amounts of precipitation. Also called micropluviometer, trace recorder.     Ombroscope An instrument which indicates the presence of precipitation. The ombroscope consists of a heated, water-sensitive surface which indicates by mechanical or electrical techniques the occurrence of precipitation.     Operational weather limits The limiting values of ceiling, visibility, and wind, or runway visual range, established as safety minima for aircraft landings and take-offs.     Operating conditions Conditions to which a device is subjected, not including the variable measured by the device. See normal operating conditions, reference operating conditions.     Operating influence The change in a performance characteristic caused by a change in a specified operating condition from reference operating condition, all other conditions being held within the limits of reference operating conditions.     Orographic precipitation Precipitation caused by the ascent of moist air over an orographic barrier such as a mountain range.     OSHA Occupational Safety and Health Administration. A regulatory office of the U.S. Department of Labor.     Owens dust recorder An instrument for rapidly obtaining samples of airborne dust; a type of dust counter. Particles pass through a cylindrical chamber, are drawn at high velocity through a narrow slit, and then impinge upon a microscope cover glass located a short distance from the slit. Analysis for quantity and size of the particles is made using a microscope. The vacuum required to operate the instrument is developed by an attached hand pump.