See evaporative opportunity.

A small balloon used to determine the height of the cloud base. The height can be computed from the ascent velocity of the balloon and the time required for its disappearance into the cloud.

Wind with a speed between 4 and 6 knots (4 and 7 mph), Beaufort scale number 2.

Abbreviation for Instrument Flight Rules, but commonly used to refer to the weather and/or flight conditions to which these rules apply, i.e. low visibility.

An instrument which measures the effective terrestrial radiation. See Angstrom pyrgeometer.

A type of wind vane having a split or V-shaped tail. The apex orients itself to the direction of the wind.

An anemometer utilizing the principle that the pitch of the aeolian tones generated by air moving past an obstacle is a function of the speed of the air. Largely a curiosity and has been put to no practical application in modem meteorology.

See calorie.

Any one of six gases, helium, neon, argon, krypton, xenon, and radon, all of whose shells of planetary electrons contain stable numbers of electrons such that the atoms are chemically inactive.

The ratio of the actual amount of water evaporated into the atmosphere to the evaporative power. Also called relative evaporation.

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.

Solar and terrestrial radiation directed upward (away From the earth's surface); outgoing radiation.

A class of rain gauge in which the level of the collected rain water is measured by the position of a float resting on the surface of the water.

An apparatus, used in studying soil temperatures, for measuring the total supply of heat during a given period.

A rainbow seen in the spray of the ocean. It is optically the same phenomenon as the ordinary rainbow.

The element directly responsive to the value of the measured variable.

A device which converts energy from one form into another, i.e. an ac generator transducer which converts the mechanical motion of anemometer cups into an electrical signal.

The ratio of the output of an instrument to the input value, i.e. a rain gauge with a sensitivity of 1 tip per 0.01"

A wave resulting from the action of wind on a water surface.

A type of recording siphon barometer. The mechanically magnified motion of a float resting on the lower mercury surface is used to record atmospheric pressure on a rotating drum.