[BNAWX] Public Information Statement

[EMWIN Server]

NOUS44 KOHX 251244
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TNZ005>011-022>034-056>066-075-077>080-093>095-260330-

PUBLIC INFORMATION STATEMENT
NATIONAL WEATHER SERVICE NASHVILLE TN
744 AM CDT TUE JUN 25 2013

..LIGHTNING SAFETY AWARENESS WEEK CONTINUES...

THE 2013 LIGHTNING SAFETY AWARENESS CAMPAIGN RUNS THROUGH JUNE
29TH. WHEN THUNDER ROARS...GO INDOORS! TODAY WE DISCUSS THE
SCIENCE BEHIND LIGHTNING.

EVERY THUNDERSTORM CONTAINS LIGHTNING. AS LIGHTNING PASSES THROUGH
THE AIR...IT RAPIDLY HEATS THE AIR TO AS HOT AS 50 THOUSAND DEGREES
FAHRENHEIT...MUCH HOTTER THAN THE SURFACE OF THE SUN. DURING A
LIGHTNING DISCHARGE...THE SUDDEN HEATING OF THE AIR CAUSES IT TO
RAPIDLY EXPAND AND CONTRACT. THIS IN TURN CAUSES A SHOCK WAVE THAT
WE HEAR AS THUNDER.

AS A THUNDERSTORM FORMS...IT PRODUCES ICE IN THE UPPER PORTION OF
THE CLOUD. THE FORMATION OF ICE IN A CLOUD IS AN IMPORTANT ELEMENT
IN THE DEVELOPMENT OF LIGHTNING. THOSE STORMS THAT FAIL TO PRODUCE
LARGE NUMBERS OF ICE CRYSTALS MAY ALSO FAIL TO PRODUCE LIGHTNING.
STRONG RISING AND SINKING MOTIONS WITHIN THE CLOUD ARE IMPORTANT
TOO...AS THEY ENHANCE COLLISIONS AMONG CLOUD PARTICLES...CAUSING A
SEPARATION OF ELECTRICAL CHARGES. POSITIVELY CHARGED ICE CRYSTALS
RISE TO THE TOP OF THE THUNDERSTORM...AND NEGATIVELY CHARGED ICE
PARTICLES AND HAILSTONES DROP TO THE MIDDLE AND LOWER PARTS OF THE
STORM.

LIGHTNING CAN OCCUR COMPLETELY WITHIN THE THUNDERSTORM CLOUD...BETWEEN
CLOUDS...OR BETWEEN THE CLOUD AND THE GROUND.  CLOUD-TO-GROUND
LIGHTNING CAN BE CATEGORIZED AS NEGATIVE FLASHES AND POSITIVE
FLASHES.  PRIOR TO A NEGATIVE FLASH...AN ALMOST INVISIBLE NEGATIVELY
CHARGED CHANNEL OF AIR FORMS NEAR THE CLOUD BASE AND SURGES DOWNWARD
NEAR THE GROUND.  AT THE SAME TIME...STREAMERS OF POSITIVE CHARGES
SHOOT UP FROM TREES...BUILDINGS...AND OTHER HIGH OBJECTS ON THE
GROUND.  WHEN THESE MEET...THE CONNECTION IS COMPLETE...AND A SURGE OF
ELECTRICAL CURRENT MOVES FROM THE GROUND TO THE CLOUD CAUSING THE
VISIBLE RETURN STROKE CALLED LIGHTNING.

POSITIVE FLASHES USUALLY OCCUR BETWEEN THE POSITIVELY CHARGED UPPER
LEVEL OF THE STORM AND THE NEGATIVELY CHARGED AREAS UNDER THE
STORM.  THE PROCESS OF A POSITIVE FLASH IS SIMILAR TO THAT OF THE
NEGATIVE FLASH.  HOWEVER...BECAUSE THE DISTANCE BETWEEN THE GROUND
AND THE STORM TOP ANVIL IS MUCH GREATER...A SIGNIFICANTLY LARGER
ELECTRIC POTENTIAL IS NEEDED TO INITIATE A POSITIVE FLASH OF
LIGHTNING.  FOR THIS REASON...POSITIVE FLASHES ARE INFREQUENT AND
WIDELY SCATTERED AROUND THE STORM...BUT THEY GENERALLY INVOLVE THE
EXCHANGE OF MUCH GREATER CHARGE AND ARE MUCH MORE DESTRUCTIVE.

TOMORROW WE WILL DISCUSS OUTDOOR LIGHTNING SAFETY.

STATISTICS...BROCHURES..MULTIMEDIA...POSTERS AND MORE IS
AVAILABLE ON THE NWS LIGHTNING SAFETY WEBSITE AT:
HTTP://WWW.LIGHTNINGSAFETY.NOAA.GOV

FOR MORE INFORMATION....CONTACT:

TOM JOHNSTONE
WARNING COORDINATION METEOROLOGIST
NWS NASHVILLE WEATHER FORECAST OFFICE
THOMAS.JOHNSTONE at NOAA.GOV
615-754-8500 X223

$$




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