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PUBLIC INFORMATION STATEMENT
NATIONAL WEATHER SERVICE MELBOURNE FL
600 AM EDT TUE JUN 22 2010
...JUNE 20ST-26TH IS LIGHTNING SAFETY AWARENESS WEEK...
THE NATIONAL WEATHER SERVICE HAS DECLARED JUNE 20ST THROUGH JUNE
26TH AS LIGHTNING SAFETY AWARENESS WEEK. THIS WEEK, THE NATIONAL
WEATHER SERVICE FORECAST OFFICE IN MELBOURNE WILL BE ISSUING
INFORMATION STATEMENTS ABOUT LIGHTNING.
TODAY'S TOPIC...LIGHTNING SCIENCE
THE AVERAGE LIGHTNING BOLT CARRIES AROUND 20 THOUSAND AMPS OF
ELECTRIC CURRENT, 100 MILLION VOLTS OF ELECTRIC POTENTIAL, AND CAN
GENERATE A TEMPERATURE IN EXCESS OF 50 THOUSAND DEGREES FAHRENHEIT
HOT ENOUGH TO MELT SAND INTO GLASS. THE AVERAGE PEAK POWER OUTPUT
OF A LIGHTNING BOLT IS ABOUT ONE TERRAWATT (OR 1,000 GIGAWATTS).
FURTHERMORE...ALL OF THIS ENERGY IS CONCENTRATED IN A CHANNEL THAT
USUALLY IS NO LARGER THAN A U.S. QUARTER. THIS GIVES LIGHTNING AN
ENERGY DENSITY THAT, AT PEAK OUTPUT, IS UNMATCHED IN THE HUMAN
WORLD...EXCEEDING EVEN THE MOST POWERFUL NUCLEAR REACTORS.
LIGHTNING OCCURS IN THE EARTH'S ATMOSPHERE 40-50 TIMES A SECOND,
FOR A TOTAL OF 1.4 BILLION STRIKES PER YEAR. LIGHTNING DETECTION
SYSTEMS ACROSS THE UNITED STATES RECORD AN AVERAGE OF 25 MILLION
CLOUD-TO-GROUND STRIKES IN THE CONTIGUOUS STATES EACH YEAR.
LIGHTNING HAS BEEN OBSERVED IN VOLCANIC ERUPTIONS, INTENSE FOREST
FIRES, HEAVY SNOW STORMS, AND LARGE HURRICANES. HOWEVER, IT MOST
OFTEN OCCURS IN THUNDERSTORMS. A THUNDERSTORM NEEDS THREE COMPONENTS
IN ORDER TO FORM: MOISTURE, INSTABILITY, AND A SOURCE OF LIFT (SUCH
AS A COLD FRONT OR A SEA BREEZE BOUNDARY). SUSTAINED RISING
MOTION WITHIN A STORM CAN ALLOW THE CLOUD TO BUILD TO A HEIGHT
50 TO 60 THOUSAND FEET...OR 6 TO 10 MILES ABOVE SEA LEVEL.
TEMPERATURES IN THE MIDDLE AND UPPER ATMOSPHERE ARE MUCH COLDER THAN
AT THE SURFACE. SUMMERTIME FREEZING LEVELS USUALLY ARE NO HIGHER
THAN 18 THOUSAND FEET. THESE COLD TEMPERATURES ALLOW ICE CRYSTALS
TO FORM IN THE HIGHER PORTIONS OF THE CLOUD. ICE IS A KEY ELEMENT
IN THE DEVELOPMENT OF LIGHTNING.
WITHIN A THUNDERSTORM, THE ICE PARTICLES VARY IN SIZE FROM SMALL
CRYSTALS TO LARGE HAILSTONES. RISING AIR CARRIES LIQUID WATER ABOVE
THE FREEZING LEVEL, WHILE SINKING AIR PUSHES FROZEN PARTICLES BELOW
THE FREEZING LEVEL. WHEN THESE COLLIDE, ELECTRONS ARE SHEARED OFF
OF THE RISING PARTICLES AND COLLECT ON THE SINKING PARTICLES (A
SIMILAR EFFECT OCCURS BY RUBBING YOUR FEET ON A CARPET). THIS IS
KNOWN AS "CHARGE SEPARATION" AND MARKS THE FIRST STEP IN THE
LIGHTNING PROCESS.
AS THE POSITIVELY CHARGED PARTICLES RISE AND THE NEGATIVELY CHARGED
PARTICLES SINK, AN ELECTRIC FIELD DEVELOPS WITHIN THE CLOUD. AT THE
SAME TIME, THE NEGATIVELY CHARGED BOTTOM OF THE CLOUD ATTRACTS A
POOL OF POSITIVELY CHARGED PARTICLES ALONG THE GROUND THAT SHADOWS
THE THUNDERSTORM WHEREVER IT GOES.
AS THE CHARGE SEPARATION CONTINUES, ENORMOUS ELECTRIC FIELDS DEVELOP
BETWEEN THE CLOUD AND THE GROUND, AS WELL AS WITHIN THE CLOUD
ITSELF. WHEN THE FIELD BECOMES STRONG ENOUGH, A NEARLY INVISIBLE
ELECTRIC CHANNEL, CALLED A "STEP-LEADER", WILL EMERGE FROM THE
BOTTOM OF THE CLOUD. IN WHAT CAN LOOSELY BE DESCRIBED AS AN
"AVALANCHE OF ELECTRONS", THE CHANNEL MOVES TOWARD THE GROUND,
SPLITTING AND FORKING IN A SERIES OF STEPS ABOUT 50 METERS IN LENGTH
AND ONE MICROSECOND IN DURATION.
WHEN THE STEP-LEADER IS WITHIN 300 FEET OF THE SURFACE, THE POOL OF
POSITIVE CHARGE WITHIN THE GROUND "REACHES" UP WITH ITS OWN CHANNEL,
CALLED A "STREAMER". WHEN THE STREAMER AND THE STEPPED LEADER
CONNECT, THE ELECTRIC CIRCUIT IS COMPLETE AND AN INTENSE ENERGY
PULSE, KNOWN AS A "RETURN STROKE" RACES UP THE CONNECTED CHANNEL.
WHAT HUMANS SEE AS LIGHTNING IS ACTUALLY TRAVELING FROM THE GROUND
INTO THE CLOUD! THIS PROCESS MAY REPEAT ITSELF SEVERAL TIMES WITHIN
A SINGLE CHANNEL, GIVING LIGHTNING ITS FLICKERING APPEARANCE. THIS
IS CALLED A "NEGATIVE STROKE", AS THE NET CHANGE OF ELECTRIC CHARGE
BETWEEN THE CLOUD AND THE GROUND IS NEGATIVE.
AS THE RETURN STROKE RACES UPWARD, IT HEATS THE AIR SURROUNDING ITS
CHANNEL TO 50 THOUSAND DEGREES FAHRENHEIT. THIS INTENSE AND
INSTANTANEOUS HEATING CAUSES THE AIR MOLECULES TO EXPAND AT A RATE
THAT EXCEEDS THE SPEED OF SOUND, A PHENOMENON KNOWN AS "EXPLOSIVE
EXPANSION". THE ACOUSTIC SHOCKWAVE THAT RESULTS IS KNOWN AS
THUNDER. LIGHTNING WILL ALWAYS PRECEDE THE THUNDER, BUT THE TIME
SEPARATION BETWEEN THE TWO WILL BE SHORTER THE CLOSER YOU ARE TO THE
STRIKE. FOR EVERY FIVE SECOND DELAY, THE LIGHTNING STRUCK ABOUT ONE
MILE AWAY.
NOT ALL LIGHTNING FORMS IN THE NEGATIVELY CHARGED AREA LOW IN THE
THUNDERSTORM. SOME LIGHTNING ORIGINATES IN THE CIRRUS ANVIL AT THE
TOP OF THE THUNDERSTORM WHERE A LARGE POOL OF POSITIVE CHARGE
EXISTS. LIGHTNING FROM THIS AREA IS CALLED "POSITIVE LIGHTNING" AS
THE NET CHANGE OF ELECTRIC CHARGE FROM CLOUD TO GROUND IS POSITIVE.
POSITIVE LIGHTNING IS PARTICULARLY DANGEROUS FOR TWO REASONS: IT
OFTEN STRIKES FAR FROM THE CENTRAL RAIN SHAFT, MORE THAN 10 MILES
RARE CASES, WHERE PEOPLE MAY NOT CONSIDER LIGHTNING A THREAT. IT
ALSO HAS A LONGER DURATION AND CAN HAVE AS MUCH AS 10 TIMES THE
PEAK CURRENT AND VOLTAGE AS "NEGATIVE LIGHTNING". THESE FACTORS
MAKE POSITIVE LIGHTNING A MUCH GREATER RISK TO LIFE AND PROPERTY.
FOR MORE INFORMATION ABOUT LIGHTNING AND LIGHTNING SAFETY...VISIT
THE LIGHTNING INFORMATION CENTER AT WEBSITE OF THE NATIONAL WEATHER
SERVICE IN MELBOURNE: HTTP://WWW.SRH.NOAA.GOV/MLB/?n=lightning
OR VISIT NOAA'S LIGHTNING SAFETY WEBPAGE AT:
HTTP://WWW.LIGHTNINGSAFETY.NOAA.GOV
$$
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