Appendix A: Glossary of Selected Meteorological Terms -Cont.

APPENDIX A

AVIATION WEATHER

2. The direction is variable and 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 and evaporating 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 forming over a body of water, and touching

the water surface; tornado or funnel cloud touching 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.

WIDESPREAD DUST Fine particles of Earth or other matter raised or suspended in the air by

the wind that may have occurred at or far away from the station.

WIND SHIFT A change in the wind direction of 45 or more in less than 15 minutes with

sustained wind speeds of ten knots or more throughout the wind shift.

A-10

Glossary of Selected Meteorological Terms