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.

A precipitation gauge consisting of a receiver in the shape of a funnel which empties into a bucket mounted upon a weighing mechanism. The weight of the catch is recorded as inches of precipitation.

A component of the wind (often using Cartesian coordinates. i.e. X and Y wind vectors). Also, an arrow representing wind velocity, drawn to point in the direction of the wind and with a length proportional to wind speed.

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.

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

The change in a performance characteristic caused by a change in a specified operating condition from reference operating condition, all other conditions being held within the limits of reference operating conditions.

A remote indicator for wind speed and direction used in conjunction with a contact anemometer and a wind vane. The indicator consists of a center light, connected to the contact anemometer, surrounded by eight equally spaced lights which are individually ...

See gust and peak gust.

The officially designated elevation of an airport above mean sea level, taken as the highest point on any of the runways of the airport. Same as airport elevation.

The measurement and computation of wind speeds and directions at various levels above the surface of the earth. Methods include pilot balloon observations, rabals, rawin or rawinsonde observations, radar tracking, or acoustic sounding.

A halo phenomenon consisting of a horizontal circle passing through the moon, corresponding to the parhelic circle through the sun. Produced by reflection of moonlight from ice crystals.

Any one of a family of circular arcs consisting of concentric colored bands, arranged from red on the inside to blue on the outside, which may be seen on a "sheet" of water drops (rain, fog, spray). The most common of the many possible is the primary rain ...

An instrument for the measurement of the radon content of the atmosphere.

The probability distribution of random errors, typically a normal distribution with a zero mean.

An instrument which measures the scattering function of particles suspended in a medium in order to determine the visual range through the medium. See visibility meter.

Dihydrogen oxide, molecular formula H2O.

The ratio of the existing amount of water vapor in the air at a given temperature to the maximum amount that could exist at that temperature. Usually expressed in percent.

A plastic molding component formed by the reaction of phenol and formaldehyde. It can be heavily reinforced or "filled" with glass fibers or other materials. Phenolics are known for their high impact strength, excellent wear characteristics, and dimension ...

Wind with a speed between 48 and 55 knots (55 and 63 mph), Beaufort scale number 10.

In general, the transformation of data from a "raw" form to some useable form. In meteorology, this often refers to the conversion of the observed value of an element to the value which it would theoretically have at some selected or standard level. The m ...