Wind with a speed below 1 knot (1 mph); Beaufort scale number 0.

An inert gas. An element found in the atmosphere to the extent of only 0.000114 percent by volume. Its molecular weight is 83.7.

A forecast of weather elements of particular interest to aviation; including ceiling, visibility, upper winds, icing, turbulence, precipitation types, and storms.

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

Liquid water at temperatures colder than freezing.

Same as luminous intensity.

Any one of a family of circular arcs consisting of concentric colored bands, arranged from red on the inside to blue on the outside, which may be seen on a "sheet" of water drops (rain, fog, spray). The most common of the many possible is the primary rain ...

A forecast of the expected stage or discharge at a specified time, or of the total volume of flow within a specified Interval of time, at one or more points along a stream.

A navigational aid used to facilitate the landing of an aircraft at an airport in instrument weather, i.e. low visibility.

A psychrometer in which the ventilation is provided by a suction fan.

The total area drained by a river and its tributaries. Same as watershed.

A synoptic code approved by the World Meteorological Organization in which the observable meteorological elements are encoded and transmitted in "words" of five numerical digits length. Often abbreviated synoptic code.

Very small precipitation drops (diameters less than 0.5 mm) that appear to float with air currents while falling in an irregular path. Unlike fog droplets, drizzle falls to the ground.

meter-An instrument for measuring the transmissivity of the atmosphere; a type of transmissometer. It consists of a constant- intensity collimated light source located at a suitable distance from a photoelectric cell. Variation in the turbidity of the atm ...

Instrument for measuring the intensity of radiant energy. Its principle is based on the variation of electrical resistance, with the incoming radiation, of one or both the metallic strips which the instrument comprises.

Navy Operational Global Atmospheric Prediction System. A global meteorological model operated by the Department of Defense.

The rate of decrease of pressure per unit distance at a fixed time.

Readily taking up and retaining moisture.

A photoelectric spectrophotometer which is used in the determination of the ozone content of the atmosphere.

The temperature at which a solid substance undergoes fusion, i.e. melts, changes from solid to liquid form. All substances have their characteristic melting points. For very pure substances the temperature range over which the process of fusion occurs is ...