Appendix A: Glossary -Cont.

METEOROLOGY FLIGHT PLANNING

APPENDIX A

PREVAILING VISIBILITY The visibility considered representative of conditions at the

station; the greatest distance that can be seen throughout at least half the horizon circle, not

necessarily continuous.

RADIOSONDE A balloon-borne instrument used to measure the temperature, pressure and

humidity aloft.

RAIN Precipitation of liquid water particles, either in the form of drops larger than .02 inch

(0.5 mm) or smaller drops which, in contrast to drizzle, are widely separated.

ROTOR CLOUD A turbulent cloud formation found in the lee of some large mountain

barriers. The air in the cloud rotates around an axis parallel to the mountain range.

RUNWAY VISUAL RANGE (RVR) An instrumentally derived value, based on standard

calibrations, that represents the horizontal distance a pilot may see down the runway from the

approach end.

SANDSTORM Particles of sand ranging in diameter from 0.008 to 1 mm that are carried aloft

by a strong wind. The sand particles are mostly confined to the lowest ten feet, and rarely rise

more than fifty feet above the ground.

SATURATED ADIABATIC LAPSE RATE A rate of decrease of temperature with height

equal to the rate at which an ascending body of saturated air will cool during adiabatic expansion.

This value will vary, but is considered to average about 1.5C per 1000 feet.

SCATTERED A layer whose summation amount of sky cover is 3/8 through 4/8.

SCHEDULED TIME OF REPORT The time a schedule report is required to be available for

transmission.

SEA-LEVEL PRESSURE The pressure value obtained by the theoretical reduction or increase

of barometric pressure to sea-level; measured in hectopascals (millibars).

SECTOR VISIBILITY The visibility in a specified direction that represents at least a 45 arc of

the horizon circle.

SHALLOW A descriptor, MI, used only to describe fog when the visibility at six feet above the

ground is 5/8 statute mile or more and the apparent visibility in the fog layer is less than 5/8

statute mile.

SHALLOW FOG Fog in which the visibility at six feet above ground level is 5/8 statute mile

or more and the apparent visibility in the fog layer is less than 5/8 statute mile.

GLOSSARY A-7