For a given cloud or cloud laver. the lowest level in the atmosphere at which the air contains a perceptible quantity of cloud particles.

Federal Aviation Administration.

The point (physical and/or electrical) where two distinct data processing elements meet.

A graphical device used for the determination of the speed of the geostrophic wind from the isobar or contour-line spacing on a synoptic chart.

A clock-driven device for recording the time of occurrence of an event or the time interval between the occurrence of events.

An instrument used to measure the amount of dew formed on a given surface.

An instrument whose calibration is determined by comparison with an absolute instrument.

A line drawn through all points on a map having the same amount of cloudiness.

Same as fogbow.

Name sometimes given to a transmissometer.

The decrease of an atmospheric variable with height, the variable being temperature, unless otherwise specified.

The general term for dry atmospheric suspensoids, including dust, haze, smoke. and sand. Compare to hydrometeor.

The elevation of the water surface in a stream as measured by a river gauge with reference to some arbitrarily selected zero datum.

See radiosonde commutator.

Temperarure at which a liquid solidifies under any given set of conditions. It may or may not be the same as the melting point or the more rigidly defined true freezing point or (for water) ice point.

A measure of the intrinsic luminous intensity emitted by a source in a given direction. Luminance is a measure only of light. The comparable term for electromagnetic radiation in general is radiance.

Any device or instrument for measuring salinity, especially one based on electrical conductivity methods.

An instrument for measuring the pressure of the atmosphere. The two principle types are aneroid and mercurial.

The distance that an observer can see vertically into a surface-based obscuring phenomenon such as fog, rain, or snow. The distance estimate must be based upon ceiling balloon ascensions or ceiling light projector measurements.

General name for a type of instrument which measures the inclination of the wind to the horizontal plane. See bivane.