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

A type of precipitation composed of unbranched crystals in the form of needles, columns, or plates. Usually has a very slight downward motion and may fall from a cloudless sky.

An instrument for measuring radiant energy. See actinometer, Dines radiometer, photometer, Tulipian radiometer.

General name for an instrument which measures the evaporation rate of water into the atmosphere. See clay atmometer, evaporation pan, evapotranspirometer, Livingston sphere, Piche evaporimeter, radio atmometer.

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

The amount of precipitation captured by a rain gauge.

An instrument for the measurement of both solar and terrestrial radiation.

In general, any object that reflects incident energy. Usually it is a device designed for specific reflection characteristics.

A hypothetical temperature characterized by a complete absence of heat and defined as 0 K, -273.15 C, or -459.67 F.

The center of an area of high pressure, usually accompanied by anticyclonic and outward wind flow. Also known as an anticyclone.

A hygrometer in which the sensitive element is a strand or strands of human hair, the length of which is a function of the relative humidity of the air.

Pertaining to the action or effect of the wind. Derived from the name of the Greek god of the winds, Aeolus.

A temperature scale with the ice point at 273 degrees and boiling point of water at 373 degrees. It is intended to approximate the Kelvin temperature scale with sufficient accuracy for many sciences, notably meteorology.

A unit of pressure used principally in oceanography. One decibar (10' dynes/cm2) equals 0.1 bar. In the ocean, hydrostatic pressure in decibars very nearly equals the corresponding depth in meters.

A line drawn through points of equal humidity on given surface.

In nautical terminology, a contraction for "weather glass" (a mercury barometer).

A balloon-borne instrument for the simultaneous measurement and transmission of meteorological data. It includes transducers for the measurement of pressure, temperature, and humidity; a modulator for the conversion of the output of the transducers to a q ...

A measure of the attenuation due to scattering, of light as it traverses a medium containing scattering particles.

The record obtained from a bolometer.

An evaporation pan in which the evaporation is measured from water in a pan floating in a larger body of water.