The ceiling is the height ascribed to the lowest layer of clouds or
obscuring phenomena when it is reported as broken, overcast, or
obscuration and not classified as "thin" or "partial." The ceiling is
termed unlimited when these conditions are not satisfied.
The greatest distance at which it is just possible to see and recognize
with the unaided eye (1) in the daytime, a prominent dark object against
the sky at the horizon, and (2) at night, a known, preferably unfocused,
moderately intense light source.
flight visibility is the average range of visibility forward from
the cockpit of an airplane in flight.
slant range visibility is the distance a pilot can see over the
nose of the airplane towards the ground. It is sometimes called approach
ground visibility is the visibility at an airport as reported by an
prevailing visibility is the distance at which objects of known
distance are visible over at least half the horizon. It is reported in
miles and fractions of miles.
Runway visual range (RVR)
The maximum distance along the runway at which the runway lights are
visible to a pilot at touchdown. Runway visual range may be determined by
an observer located at the end of the runway, facing in the direction of
landing, or by means of a transmissometer installed near the end of the
VMC and IMC
Visual meteorological conditions (VMC)indicates that visibility, distance
from cloud and ceiling are equal to or better than the minimum under which
flight according to the visual flight rules (VFR) may be conducted.
Instrument meteorological conditions (IMC)indicates that visibility,
distance from cloud and ceiling are below minima and flight can be
conducted only under instrument flight rules (IFR).