General term for an instrument which records the vertical electric current in the atmosphere.

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

Precipitation falling from a cloud, usually in wisps or streaks, but evaporating before it reaches the ground.

Temperature to which absolutely dry air would have to be brought in order for it to have the same density as moist air, considered at the same pressure.

The greatest distance at which it is just possible to see and recognize with the unaided eye (1) in the daytime, a prominent dark object against the sky at the horizon, and (2) at night, a known, preferably unfocused, moderately intense light source.

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.

Electromagnetic radiation lying within the wavelength interval to which the human eye is sensitive, the spectral interval from approximately 0.4 to 0.7 microns (4000 to 7000 angstroms). Bounded on the short-wavelength end by ultraviolet radiation and on t ...

The maximum distance, usually horizontally, at which a given object or light source is just visible under particular conditions of transmittance and background luminance.

Routine broadcast of meteorological information for aircraft in flight.

A thermometer used in aircraft which automatically corrects for adiabatic and frictional temperature rises by imparting a rotary motion to the air passing the thermal sensing element.

A measure of the amount of "spin" (or rotation) in the atmosphere.

The stage on a fixed river gauge at which it is necessary to begin issuing warnings or river forecasts if adequate precautionary measures are to be taken before flood stage is reached.

Dihydrogen oxide, molecular formula H2O.

Balance of the water resources of a region, comparing precipitation and inflow with outflow, evaporation, and accumulation.

See hydrologic accounting.

The depth of water that would result from the melting of snow or ice, assuming measurement on a horizontal surface and no infiltration or evaporation.

The depth below which the ground is saturated with water. No water table exists if the ground water is confined by an overlying impermeable stratum, as in the case of artesian ground water.

Any twelve-month period, usually selected to begin and end during a relative dry season. Used a basis for processing streamflow and other hydrologic data. The period from October 1 to September 30 is widely used in the U.S.

An absolute pyhrliometer, developed by C.G. Abbott, in which the radiation-sensing element is a blackened water-calorimeter.

A device for obtaining a continuous record of stage at a point on a stream. The most common recorders consist of a float-actuated pen which traces a record on a clock driven chart.