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

See hurricane beacon.

See radiosonde commutator.

A device which automatically records the water equivalent of snow on a given surface as a function of time. A small sample of a radioactive salt is placed in the ground in a shielded collimator which directs a beam of radioactive particles upwards. A Geig ...

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 ...

Abbreviation for precipitation-evaporation ratio.

A vector term which includes both wind speed and wind direction.

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.

A local variation in the wind direction or speed. This condition can present danger to aircraft, especially at landing, when a sudden shift from headwind to tailwind can cause a rapid loss of airspeed and lift.

The true freezing point of water. The temperature at which a mixture of air-saturated pure water and pure ice may exist in equilibrium at a pressure of one standard atmosphere.

The length of air flow past a wind vane required for the vane to respond to 50 percent of a step change in wind direction. Expressed in feet or meters and calculated from delay time times wind tunnel speed.

A sudden brief increase in the speed of the wind, followed by a lull or slackening. Compare to peak gust.

General term for any device that measures precipitation: principally a rain gauge or snow gauge.

A measure of long-term precipitation effectiveness. The ratio of the normal annual rainfall to the normal annual evaporation.

Automated Weather Observing Station. A self-contained weather station designed to make aviation weather observations without operator involvement.

An instrument that automatically records pressure and temperature.

The actual time during which physical events take place.

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.

A semiconductor which exhibits rapid and extremely large changes in resistance for relatively small changes in temperature.

An instrument used to measure the water vapor content of the air.