The difference between temperature measurements taken at two significant levels above the ground. Temperatures at 10 and 40 meters are commonly used.

A hygrometer which determines the amount of precipitable moisture in a given region of the atmosphere by measuring attenuation of radiant energy caused by the absorption bands of water vapor.

A basic equation in night visual range theory, relating the illuminance of a point source of light to distance and the transmissivity of the atmosphere.

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

The total area drained by a river and its tributaries. Same as watershed.

A clock-driven instrument mounting which automatically and continuously points in the direction of the sun. It is used with a pyrheliometera,hen continuous direct solar radiation measurements are required.

The amount of sky covered or concealed by clouds or obscuring phenomena. It is reported in tenths, so that 0.0 indicates a clear sky and 1.0 (or 10/10) indicates a completely covered sky. The following classifications are used in aviation weather observat ...

A radar which is used to obtain the azimuth elevation, and slant range of an airborne target.

Any one of numerous devices for the measurement of either speed alone or of both direction and speed (set and drift) in flowing water.

Facing away from the wind.

February 2nd. In American folklore, a day that is popularly supposed to provide the key to the weather for the remainder of the winter. Specifically, if the ground-hog upon emerging from its hole casts a shadow, it will return underground, thereby forebod ...

A thermodynamic change of state in a system in which there is no transfer of heat or mass across the boundaries of the system. In an adiabatic process, compression always results in warming, expansion in cooling. Compare to diabatic process.

An instrument used to measure and record earthquake vibrations and other earth tremors.

Ragged low clouds, usually stratus fractus. Most often applied when such clouds are moving rapidly beneath a layer of nimbostratus.

A fixed support for mounting maximum and minimum thermometers of the liquid-in-glass type. The support holds the thermometers at the correct operating attitude and also permits their rotation for resetting when desired.

The pressure exerted by the atmosphere as a consequence of gravitational attraction exerted upon the "column" of air lying directly above the point in question.

Fine dust or salt particles dispersed through a portion of the atmosphere; a type of lithometer. The particles are so small they cannot be felt or seen with the naked eye. Many haze formations are caused by the presence of an abundance of condensation nuc ...

Temperarure at which a liquid solidifies under any given set of conditions. It may or may not be the same as the melting point or the more rigidly defined true freezing point or (for water) ice point.

For a given cloud or cloud laver. the lowest level in the atmosphere at which the air contains a perceptible quantity of cloud particles.

An instrument used for measuring geometric angles. See radio direction-finder.