Appendix A: Glossary of Selected Meteorological Terms -Cont.

AVIATION WEATHER

APPENDIX A

DEW POINT The temperature to which a given parcel of air must be cooled at constant

pressure and constant water-vapor content in order for saturation to occur.

DISPATCH VISUAL RANGE A visual range value derived from an automated visibility

sensor.

DRIZZLE Fairly uniform precipitation composed exclusively of fine drops (diameter less than

0.02 inch or 0.5 mm) very close together. Drizzle appears to float while following air current,

although unlike fog droplets, it falls to the ground.

DRY ADIABATIC LAPSE RATE The rate of decrease of temperature with height,

approximately equal to 3C per 1000 feet. This is close to the rate at which an ascending body of

unsaturated air will cool by adiabatic expansion.

DUSTSTORM An unusual weather condition, frequently severe, characterized by strong winds

and dust-filled air over an extensive area.

FEW A layer whose summation amount of sky cover is 1/8 through 2/8.

FIELD ELEVATION The elevation above sea level of the highest point on any of the runways

of the airport.

FOG A visible aggregate of minute water particles (droplets) which are based at the Earth's

surface and reduce horizontal visibility to less than 5/8 statute mile and, unlike drizzle, it does

not fall to the ground.

FREEZING A descriptor, FZ, used to describe drizzle and/or rain that freezes on contact with

the ground or exposed objects, and used also to describe fog that is composed of minute ice

crystals.

FREEZING DRIZZLE Drizzle that freezes upon impact with the ground, or other exposed

objects.

FREEZING FOG A suspension of numerous minute ice crystals in the air or water droplets at

temperatures below 0Celsius based at the Earth's surface, which reduces horizontal visibility. It

is also called ice fog.

FREEZING PRECIPITATION Any form of precipitation freezing on impact and forming a

glaze on the ground or exposed objects.

FREEZING RAIN Rain freezing on impact and forming a glaze on the ground or exposed

objects.

FROZEN PRECIPITATION Any form of precipitation that reaches the ground in solid form

(snow, small hail and/or snow pellets, snow grains, hail, ice pellets, and ice crystals).

Glossary of Selected Meteorological Terms A-3