Public Information Statement
Issued by NWS Duluth, MN

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MNZ010>012-018>021-025-026-033>038-WIZ001>004-006>009-250500-

PUBLIC INFORMATION STATEMENT
NATIONAL WEATHER SERVICE DULUTH MN
738 AM CDT TUE JUN 24 2014

...LIGHTNING SAFETY WEEK DAY TWO...

...TODAYS TOPIC IS THE SCIENCE OF LIGHTNING...

AT ANY GIVEN MOMENT IT IS ESTIMATED THERE ARE EIGHTEEN HUNDRED
THUNDERSTORMS IN PROGRESS SOMEWHERE ON EARTH AND SIXTEEN MILLION
THUNDERSTORMS EACH YEAR. SCIENTISTS STUDYING LIGHTNING HAVE A BETTER
UNDERSTANDING TODAY OF THE PROCESS THAT PRODUCES LIGHTNING AND THERE
IS STILL MORE TO LEARN.

THE PRESENCE OF ICE AND LIQUID WATER IN THE SAME REGION OF A
CLOUD IS CRITICAL IN THE DEVELOPMENT OF LIGHTNING. CLOUDS THAT DO
NOT HAVE LARGE QUANTITIES OF ICE AND WATER COLLOCATED FAIL TO
PRODUCE LIGHTNING. IN A THUNDERSTORM...THE ICE PARTICLES VARY IN
SIZE FROM SMALL CRYSTALS TO LARGER HAILSTONES. DUE TO THE RISING
AND SINKING MOTIONS WITHIN STORMS...THERE ARE NUMEROUS COLLISIONS
BETWEEN ICE AND WATER PARTICLES OF DIFFERENT SIZES. THE SMALLER
PARTICLES TEND TO TAKE ON A POSITIVE CHARGE AND ARE LIFTED HIGH
INTO THE CLOUD...WHILE THE HEAVIER...NEGATIVELY CHARGED PARTICLES
FALL TOWARD THE EARTH.

THROUGH THIS PROCESS THE UPPER PORTIONS OF THE CLOUD BECOME
POSITIVELY CHARGED AND THE LOWER PORTIONS OF THE CLOUD BECOME
NEGATIVELY CHARGED...INCREASING THE ELECTRICAL POTENTIAL WITHIN
THE CLOUD. AS THE NEGATIVELY CHARGED PARTICLES FALL TOWARD THE
EARTH...THE EARTH RESPONDS BY GENERATING A SLIGHT POSITIVE CHARGE
NEAR THE SURFACE. OVER TIME...THE ELECTRIC POTENTIAL INCREASES TO
THE POINT WHERE IT CAN OVERCOME THE INSULATING EFFECT OF THE AIR AND
CREATES A SPARK WE CALL LIGHTNING.

THIS SPARK CAN EXTEND FROM THE CLOUD TO THE GROUND...IN THE CASE
OF CLOUD TO GROUND LIGHTNING...OR BETWEEN THE TWO CHARGED REGIONS
WITHIN THE CLOUD...CALLED IN CLOUD LIGHTNING.

ONE OF THE CHARGED AREAS IN THE CLOUD WILL SEND OUT A FEELER
TOWARD AN AREA WITH THE OPPOSITE CHARGE. THAT OTHER AREA WILL
SEND A FEELER BACK. THIS PROCESS CONTINUES BACK AND FORTH...IN
SMALL STEPS...UNTIL THE TWO FEELERS MEET AND THE ELECTRICITY
TRANSFER BEGINS. THE FEELER PROCESS OCCURS OVER A FEW FRACTIONS
OF A SECOND AND IS INVISIBLE TO THE HUMAN EYE. THE ELECTRICITY
TRANSFER OCCURS OVER SEVERAL SHORT BURSTS THROUGH THE CHANNEL OR
AS A LONGER...SUSTAINED BURST. THE ENTIRE PROCESS IS OVER WITHIN
A FEW SECONDS.

THE CHANNEL THROUGH WHICH THE LIGHTNING TRAVELS IS ROUGHLY THE
WIDTH OF THE PENCIL. THE AIR WITHIN THIS SPACE HEATS VERY RAPIDLY
TO AROUND FIFTY THOUSAND DEGREES FAHRENHEIT...FIVE TIMES HOTTER
THAN THE SURFACE OF THE SUN. AS THE AIR HEATS IT EXPANDS. AFTER
THE TRANSFER OF ELECTRICITY IS FINISHED THE AIR COOLS JUST AS
QUICKLY AND COLLAPSES. THE RAPID EXPANSION AND COLLAPSE OF THE
AIR IS WHAT WE HEAR AS THUNDER.

A BOLT OF LIGHTNING CAN REACH OUTWARD TEN MILES FROM THE PARENT
THUNDERSTORM. THIS IS WHY WE SAY IF YOU ARE CLOSE ENOUGH TO HEAR
THUNDER...YOU ARE CLOSE ENOUGH TO BE STRUCK BY LIGHTNING.

ON DAY THREE...WE WILL DISCUSS BEST PRACTICES FOR LIGHTNING
SAFETY.

$$

HUYCK












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