Aeronautical Terms beginning with U

Undercast

A cloud layer of ten-tenths (1.0) coverage (to the nearest tenth) as viewed from an observation point above the layer.

Unlimited ceiling

A clear sky or a sky cover that does not meet the criteria for a ceiling.

Unstable

Instability. A general term to indicate various states of the atmosphere in which spontaneous convection will occur when prescribed criteria are met; indicative of turbulence.

Updraft

A localized upward current of air.

Upper front

A front aloft not extending to the earth’s surface.

Upslope fog

Fog formed when air flows upward over rising terrain and is, consequently, adiabatically cooled to or below its initial dew point.