Same as constant-level balloon.

See calorie.

An atmometer consisting of a porous porcelain or ceramic container connected to a calibrated reservoir filled with distilled water. Evaporation is determined by the depletion of water in the reservoir.

A unit of energy equal to 101 ergs or to 0.2389 calories.

Interational thermometric scale on which the freezing point of water equals 0

A buoyant balloon kept from rising freely by means of a line secured to a point on the ground, as opposed to a free balloon. See kytoon.

A relatively small-scale, upward moving current of air.

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.

Infrared radiation emitted by or being propagated through the atmosphere.

A thermograph in which ventilation is provided by a suction fan.

A thermoelectric thermometer used for measuring air temperature. The name is derived from the fact that the reference thermocouple is placed in an insulated bottle.

Turbulence encountered by aircraft when flying through air space devoid of clouds. Thermals and wind shear are the main causes.

Thermometer used for measuring the highest temperature attained during a given interval of time, for example, a day.

Same as wind sock.

Temperature of equilibrium between the liquid and vapor phases of a substance at a given pressure.

The older name for the Celsius temperature scale. Officially abandoned by international agreement in 1948, but still in common use.

A decrease in the central pressure of a pressure system. Usually applied to a low rather than to a high.

Abbreviation for visual flight rules, but commonly used to refer to the relatively favorable weather and/or flight conditions to which these rules apply.

Difference between the instantaneous wind velocity at a point and the mean wind velocity taken over a given time interval. Also called fluctuation velocity.

General term for an instrument used to make direct measurements of visual range or measurements of the physical characteristics of the atmosphere which determine the visible range.