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 An instrument-derived value, based on standard calibrations,
representing 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 carried aloft by a
strong wind. The sand particles are mostly confined to the lowest ten feet, and rarely rise more
than 50 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 1.5°C. 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 scheduled report is required to be available for
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 45º 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
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.
SHOWER(S) A descriptor, SH, used to qualify precipitation characterized by the suddenness
with which they start and stop, by the rapid changes of intensity, and usually by rapid changes in
the appearance of the sky.
SIGNIFICANT CLOUDS Cumulonimbus, cumulonimbus mammatus, towering cumulus,
altocumulus castellanus, and standing lenticular or rotor clouds.
SKY CONDITION The state of the sky in terms of such parameters as sky cover, layers and
associated heights, ceiling, and cloud types.
SKY COVER The amount of the sky covered by clouds or partial obscurations in contact with
Glossary of Selected Meteorological Terms A-7