SUMMATION PRINCIPLE This principle states the sky cover at any level is equal to the
summation of the sky cover of the lowest layer, plus the additional sky cover present at all
successively higher layers up to and including the layer being considered.
SURFACE VISIBILITY The prevailing visibility determined from the usual point of
SYNOPTIC CHART A chart, such as the ordinary weather map, which shows the distribution
of meteorological conditions over an area at a given moment.
THUNDERSTORM A descriptor, TS, used to qualify precipitation produced by a
detected by lightning detection systems.
TIME OF OCCURRENCE A report of the time weather begins and ends.
TORNADIC ACTIVITY The occurrence or disappearance of tornadoes, funnel clouds, or
TORNADO A violent, rotating column of air touching the ground or funnel cloud touching the
ground (see funnel cloud and water spout).
TOWER VISIBILITY The prevailing visibility determined from the airport traffic control
tower when the surface visibility is determined from another location.
TOWERING CUMULUS A descriptive term for a cloud with generally sharp outlines and
with moderate to great vertical development, characterized by its cauliflower or tower
UNKNOWN PRECIPITATION Precipitation type reported if the automated station detects the
occurrence of light precipitation but the precipitation discriminator cannot recognize the type.
VARIABLE CEILING A ceiling of less than 3000 feet, which rapidly increases or decreases in
height by established criteria during the period of observation.
VARIABLE LAYER AMOUNTS A condition when the reportable amount of a layer varies by
one or more reportable values during the period it is being evaluated (variable sky condition).
VARIABLE PREVAILING VISIBILITY A condition when the prevailing visibility is less
than three statute miles and rapidly increases and decreases by 1/2 mile or more during the
period of observation.
VARIABLE WIND DIRECTION A condition when:
1. The wind direction fluctuates by 60º or more during the two minute evaluation period and
wind speed is greater than six knots; or
Glossary of Selected Meteorological Terms A-9