Appendix A: Glossary -Cont.

APPENDIX A

METEOROLOGY FLIGHT PLANNING

UNKNOWN PRECIPITATION Precipitation type that is reported if the automated station

detects the occurrence of light precipitation but the precipitation discriminator cannot recognize

the type.

VARIABLE CEILING A ceiling of less than 3000 feet which rapidly increases or decreases in

height by established criteria during the period of observation.

VARIABLE LAYER AMOUNTS A condition when the reportable amount of a layer varies by

one or more reportable values during the period it is being evaluated (variable sky condition).

VARIABLE PREVAILING VISIBILITY A condition when the prevailing visibility is less than

three statute miles and rapidly increases and decreases by 1/2 mile or more during the period of

observation.

VARIABLE WIND DIRECTION A condition when (1) the wind direction fluctuates by 60 or

more during the two-minute evaluation period and the wind speed is greater than six knots; or (2)

the direction is variable and the wind speed is six knots or less.

VERTICAL VISIBILITY A subjective or instrumental evaluation of the vertical distance into a

surface-based obscuration that an observer would be able to see.

VICINITY A proximity qualifier, VC, used to indicate weather phenomena observed between

five and ten statute miles of the usual point of observation but not at the station.

VIRGA Visible wisps or strands of precipitation falling from clouds that evaporate before

reaching the surface.

VISIBILITY The greatest horizontal distance at which selected objects can be seen and

identified or its equivalent derived from instrumental measurements.

VOLCANIC ASH Fine particles of rock powder that originate blown out from a volcano and

that may remain suspended in the atmosphere for long periods. The ash is a potential hazard to

aircraft operations and may be an obscuration.

VOLCANIC ERUPTION An explosion caused by the intense heating of subterranean rock

which expels lava, steam, ashes, etc., through vents in the earth's crust.

WATERSPOUT A violent, rotating column of air that forms over a body of water, and touches

the water surface; tornado or funnel cloud that touches a body of water (see funnel cloud and

tornado).

WELL-DEVELOPED DUST/SAND WHIRL An ensemble of particles of dust or sand,

sometimes accompanied by small litter, raised from the ground in the form of a whirling column

of varying height with a small diameter and an approximately vertical axis.

A-10 GLOSSARY