A warm, dry wind on the lee side of a mountain range, the warmth and dryness due to adiabatic compression upon descent.

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.

A basic equation in daytime visual range theory, relating the apparent luminance of a distant black object, the apparent luminance of the background sky above the horizon, and the extinction coefficient of the atmosphere, or the air layer near the ground. ...

Instrument for measuring the intensity of radiant energy. Its principle is based on the variation of electrical resistance, with the incoming radiation, of one or both the metallic strips which the instrument comprises.

A rain gauge which is placed under trees or foliage to determine the rainfall in that location. By comparing this catch with that from a rain gauge set in the open. the amount of rainfall which has been intercepted by foliage can be determined.

Radio Direction Finder.

Dihydrogen oxide, molecular formula H2O.

Any sudden and heavy rain, almost always of the shower type.

The value of atmospheric pressure to which the scale of a pressure altimeter is set so as to indicate airport elevation. The altimeter setting is included as part of an aviation weather observation.

A pyrheliometer of the bimetallic type used to measure the intensity of direct solar radiation.

Same as instrument shelter.

See mote.

The direction, with respect to true north, from which the wind is blowing. Distinguish from magnetic wind direction. In all standard upper-air and surface weather observations, it is true wind direction that is reported.

Precipitation composed of white or translucent ice crystals, chiefly in complex branched hexagonal forms.

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

A rotation anemometer in which the axis of rotation is horizontal. The instrument has either flat vanes (as in the air meter) or helicoidal vanes (as in the propeller anemometer). The relation between wind speed and angular rotation is almost linear.

Diffuse solar radiation which has the same intensity in all directions.

An increase in the central pressure of a pressure system; opposite of a deepening. More commonly applied to a low rather than a high.

Amount of solar radiation incident, per unit area and time, on a surface which is perpendicular to the radiation and is situated at the outer limit of the atmosphere, the earth being at its mean distance from the sun. It equals approximately 2.00 ly/ min ...

The transmission of data collected at a remote location over communications channels to a central station.