In aviation terminology, route or terminal weather conditions of sufficiently low visibility to require the operation of aircraft under instrument flight rules.

An optical instrument which consists of a sighting telescope mounted so that it is free to rotate around horizontal and vertical axes, with graduated scales so that the angles of rotation may be measured. Used to observe the motion of a pilot balloon.

The maximum distance along the runway at which the runway lights are visible to a pilot at touchdown. Runway visual range may be determined by an observer located at the end of the runway, facing in the direction of landing, or by means of a transmissomet ...

Wind with a speed between 22 and 27 knots (25 and 31 mph); Beaufort scale number 6.

Wind with a speed between 11 and 16 knots (13 and 18 mph); Beaufort scale number 4.

Formation of a single water drop by the union of two or more colliding drops.

An instrument for measuring the frost point of the atmosphere.

A system of physical units based on the use of the centimeter, gram. and the second as elementary quantities of length. mass. and time.

A line drawn through geographical points where a given seasonal biological event occurs on the same date.

An atmospheric phenomenon, other than clouds, which obscures a portion of the sky from the point of observation. Also called obscuration.

The portion of the precipitation on the land which ultimately reaches the streams. especially the water from rain or melted snow that flows over the surface.

A contact anemometer connected to an electrical circuit which is so arranged that the average wind speed is indicated.

A numbering system using a base number of 16 and including the ten decimal digits (0 to 9) along with six alpha digits (A to F). Thus. a digit is available to represent each of the possible values of a 4-bit binary digit.

Negative Vorticity Advection.

A type of instrument shelter. It is a wooden box painted white with double louvered sides and mounted on a stand four feet above the ground.

Name sometimes given to a transmissometer.

Abbreviation for binary digit. The smallest unit of information, equal to one binary decision, i.e. 1/0, on/off, yes/no.

A modification of the dew cell used in radiosonde equipment.

A unit of mass numerically equal to the molecular weight of the substance. The gram-mote or gram-molecule is the mass in grams numerically equal to the molecular weight, i.e. a gram-mole of oxygen is 32 grams.

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